What is the rationality behind reproduction in context of humans?

It is not a rational thing. Reproduction is an instinct that we share with all other animals.

From an evolutionary view, maybe there was a set of humans who didn't like to reproduce. Guess what happened, their genes disappeared! Those who liked to reproduce (liked sex and kids as-well) propagated and so did their genes, thus the off-springs had similar inclinations/instincts (to reproduce). Also, people try to find meaning in their lives, most people think of their kids as their meaning of life/most important thing so it gives them a purpose. Look at the bond between a mother and a child to verify this.

We can actually take this way beyond scope. Such as what is the meaning of life? Why do we care if we reproduce or not? Why do we care if we survive? That's something we cannot answer - not yet. Given that, I think we can never truly understand the rational behind reproduction, other than the fact that humans, like other animals, have to fulfill their desires - with sex being one of them.

I am not sure if there is a clear answer for this, I have thought a long time about it. This is a rather philosophical question. The answer I hear frequently when I raise this question is that people feel they continue to live in their children after their death. This might relate to the legacy that was mentioned in earlier answers to this question. Myself I tend to think that it is also a way of mitigating risk. This would also explain why sometimes family encourages to reproduce.