I watch this interesting clip today (https://m.youtube.com/watch?v=FLsWif6-Jfg), but I don't really understand the joke that he makes, which is:
A: We're going to mountain-climbing tomorrow, but I don't think you should climb to fast.
B: You don't think I should climb to fast? Why do you say that?
A: Well if you peak too soon, there will be all downhill from there.
Could you please help me to understand what makes people laugh in this joke? I think it must be funny, but I can't really get it.