But, we know that isn't the case. Even today we see people that seem to want their eternal damnation, but it is only because they do not understand the immense repercussions of such. Once a woman told me, "I'm driving the bus to hell and I'll give anyone a ride!" And even though I know she meant it, I also know that she didn't really understand the meaning of her words.
I believe that many people, and even Israel of old, willfully turn their backs on God because they believe that the strait gate (cf. Matthew 7:13-14) is the gate of joylessness and despair. They see following God as a burden they aren't willing to bear! At least, I know that was the motivation behind the statement of the self-proclaimed bus driver mentioned above.
But God commands that His followers do so with joy in their hearts (cf. Deuteronomy 28:47, Philippians 4:4).
Because Pessimism is Powerless
Pessimism lives on these verses. Because the joyless disciple wants to relish the fact that he will stumble (cf. Romans 3:23). And, when he faces these temptations, persecutions and doubts, he will give up just like the Israelites did. He is powerless to them because of his lack of joy!
Because Joy Takes Commitment
God doesn't want the Sunday and (maybe) Wednesday Christians! He wants the 25/8 Christians that live His Way regardless of the time of day, circumstance or outcome. He wants His people to be wholly committed to the cause. And therefore, He wants His saints to work tirelessly for the joy set before them even in the shadow of despair just like He did (cf. Hebrews 12:2). His commands of joyful Christianity are commands of commitment!
Because It's the End That Counts
What does it truly matter if we suffer things in this life when it is the afterlife for which God created us (cf. Ecclesiastes 12:13)?
Joy is vital to the Christian walk because it allows us to see past the present and see to the future (2 Corinthians 4:18).
If we will strive to follow Him with joy, we won't give up we won't fail and we won't fall because we will know that pessimism not only gives us no power but that it sucks what little power we did have away. Just like in the case of the Israelites. They had the power to overcome their situations, they had the power to stay faithful to God Almighty. But their pessimism destroyed their chances all because they failed to follow Him with joy in their hearts. And so they were doomed.