Question:
What’s to stop a Christian from killing himself to be with Christ?

Answer:
It is true that the more a Christian gets to know the Lord Jesus Christ, the more he will want to be with Him (and soon every believer will be with Him forever in the glory), but it would be a very selfish thing to kill oneself in order to be with Christ.

One who would do this is not considering his loved ones, his friends, and he certainly is not considering what the Lord desires. How could a Christian think that he could be pleasing Christ by killing himself in order to be with Him?
Killing ourselves (or someone else) is sin, and God hates sin. “Thou shalt not kill” (Exodus 20:13). Because of all that Christ has done for us, every Christian should be concerned, not with what we want, but with what Christ desires. We should ask: “Is it Christ’s will for me to die now?” We should always seek to do “the purpose of Him,” and know “the counsel of His own will” (Ephesians 1:11).
Such thoughts may come when a person is overcome with trials. We should realize that pain, suffering, and shame are gifts that God gives to every Christian (Philippians 1:29), and we should count it a privilege to bear them (Acts 5:41), and “rejoice in [our] sufferings” (Colossians 1:24). The Lord uses trials as tools to teach us valuable lessons such as: how to get rid of self-will, to trust the Lord, and to train us for His presence in the glory. “Forasmuch then as Christ hath suffered for us in the flesh, arm yourselves likewise with the same mind…that he no longer should live the rest of his time in the flesh to the lusts of men, but to the will of God” (1 Peter 4:1,2).
Elijah told the Lord he wanted to die. “It is enough; now, O Lord, take away my life” (1 Kings 19:4). But the Lord did not take his life away right then; He had other things planned for Elijah. “Whom the Lord loveth He chasteneth [trains]….Now no chastening for the present seemeth to be joyous, but grievous: nevertheless afterward it yieldeth the peaceable fruit of righteousness unto them which are exercised thereby” (Hebrews 12:6, 11).