Her father is rejecting her suitor because he is a mujaahid and may die in battle

Firstly: 

Marriage is a noble, legitimate bond that is encouraged by our religion in several places in the Book of Allaah and the Sunnah of His Messenger. One of the most important factors in this bond is to make a good choice when selecting a husband or wife. The Prophet (peace and blessings of Allaah be upon him) said: “Women are married for four things: for their wealth, their lineage, their beauty and their religious commitment. Choose the one who is religiously committed, may your hand be rubbed with dust [i.e., may you prosper].” Narrated by al-Bukhaari, 4802; Muslim, 1466. 

And he said, addressing the guardians (walis) of women: “If there comes to you with a proposal of marriage one with whose religious commitment and attitude you are pleased, then marry [your daughter or female relative under your care] to him, for if you do not do that, there will be fitnah (tribulation) on earth and widespread corruption.” Narrated by al-Tirmidhi, 1084; Ibn Maajah, 1967. Classed as saheeh by Shaykh al-Albaani in al-Silsilah al-Saheehah, 1022. 

Moreover jihad is one of the greatest kinds of good deeds, and it is the pinnacle of Islam. Those who fight in jihad are among the best of the slaves of Allaah, and are among His party of the successful. The virtue of jihad and the mujaahideen would take too long to explain here. 

Secondly: 

Going for jihad does not mean that death is certain. Allaah has created all beings and has decreed their provision and life spans. It is strange that a Muslim would think that going for jihad in the battle field means that he will be killed there, and that if he does not go for jihad he will escape death. This is a mistaken notion, because jihad does not hasten the hour of a man’s death, and not going for jihad does not delay it. A person may spend his whole life in jihad and not be killed in battle. One of the best examples of that is the great Sahaabi Khaalid ibn al-Waleed (may Allaah be pleased with him) who spent most of his life in the battlefield, then he died in his bed, whilst thousands of people die every minute in their homes or places of work or on the street. Allaah says (interpretation of the meaning): 

“And if Allaah were to seize mankind for their wrongdoing, He would not leave on it (the earth) a single moving (living) creature, but He postpones them for an appointed term and when their term comes, neither can they delay nor can they advance it an hour (or a moment)”

[al-Nahl 16:61] 

“No person knows what he will earn tomorrow, and no person knows in what land he will die. Verily, Allaah is All‑Knower, All‑Aware (of things)”

[Luqmaan 31:34]

Allaah tells us of those who think like this, and He refutes them: 

“O you who believe! Be not like those who disbelieve (hypocrites) and who say to their brethren when they travel through the earth or go out to fight: ‘If they had stayed with us, they would not have died or been killed,’ so that Allaah may make it a cause of regret in their hearts. It is Allaah that gives life and causes death. And Allaah is All‑Seer of what you do”

[Aal ‘Imraan 3:156 – interpretation of the meaning]

Ibn Katheer said:  

Allaah forbids His believing slaves to imitate the kuffaar in their corrupt belief which is indicated by what they said concerning their brethren who died on journeys or in war, that if they had not travelled or gone to war, what befell them would not have befallen them. Allaah says “O you who believe! Be not like those who disbelieve (hypocrites) and who say to their brethren” i.e., of their brethren; “when they travel through the earth” i.e., for the purpose of trading etc; “or go out to fight” i.e., on military campaigns; “If they had stayed with us” i.e., in our land; “they would not have died or been killed” i.e., they would not have died on the road or been killed in battle. The words, “so that Allaah may make it a cause of regret in their hearts” mean, He has created this belief in their hearts to increase their sense of grief for their people who died or were slain. 

Then Allaah says, refuting them:

“It is Allaah that gives life and causes death” meaning, creation is in His hand, and to Him all things return. No one lives or dies except by His will and decree, and there can be no increase or decrease in the life span of anyone except by His will and decree.  

Tafseer Ibn Katheer, 1/420. 

And he said: 

Just as caution does not change the will and decree of Allaah, so too fleeing from jihad or avoiding it does not hasten or delay the appointed time of death. Rather life span is decided and provision is allocated. And it cannot be increased or decreased, as Allaah says (interpretation of the meaning): 

“(They are) the ones who said about their killed brethren while they themselves sat (at home): ‘If only they had listened to us, they would not have been killed.’ Say: ‘Avert death from your ownselves, if you speak the truth’”

[Aal ‘Imraan 3:168] 

“They say: ‘Our Lord! Why have you ordained for us fighting? Would that you had granted us respite for a short period?’ Say: ‘Short is the enjoyment of this world. The Hereafter is (far) better for him who fears Allaah, and you shall not be dealt with unjustly even equal to the Fateela (a scalish thread in the long slit of a date stone).

Wheresoever you may be, death will overtake you even if you are in fortresses built up strong and high!’”

[al-Nisa’ 4:77-78]

And we have narrated from the commander of the army, the leader of the troops, the guardian of Islam, the sword of Allaah unsheathed against His enemies, Abu Sulaymaan Khaalid ibn al-Waleed (may Allaah be pleased with him) that he said – when he was about to die – “I have seen such and such battles, and there is no part of my body that has not been struck by arrows, stabbed or hit, but here I am about to die in my bed like a camel. May the eyes of cowards know no sleep” – meaning that he was sad that he had not been killed in war, and was grieved that he was going to die in his bed. 

Tafseer Ibn Katheer, 1/300.  

Thirdly:  

From the above it is clear that what your father says about not wanting to see you widowed in his lifetime is mistaken, because if Allaah has decreed that for you, it will happen even if you marry someone who is not a mujaahid. His refusing this man because he is preparing himself for jihad is not an acceptable justification according to sharee’ah. But if there are other reasons that have to do with his attitude and religious commitment, then the father can refuse this marriage. 

Fourthly: 

Our advice to you is to obey your father. You do not have the right to marry without the permission of your guardian. The wise sharee’ah regards the marriage of a woman without the permission of her guardian as invalid. 

(a) It was narrated that Abu Moosa said: The Prophet (peace and blessings of Allaah be upon him) said: “There is no marriage without a guardian.”

Narrated by al-Tirmidhi, 1101; Abu Dawood, 2085; Ibn Maajah, 1881. Classed as saheeh by Shaykh al-Albaani in Saheeh al-Tirmidhi, 1/318. 

(b) It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: The Messenger of Allaah (S) said: “Any woman who gets married without the consent of her guardian, her marriage is invalid, her marriage is invalid, her marriage is invalid,” but if the marriage has been consummated then the mahr belongs to her because she has allowed him to be intimate with her. And if she has no guardian then the ruler is the guardian of the one who has no guardian.

Narrated by al-Tirmidhi, 1102; Abu Dawood, 2083; Ibn Maajah, 1879. Classed as hasan by al-Tirmidhi and as saheeh ibn Ibn Hibbaan (9/384) and al-Haakim (2/183). 

And Allaah knows best.