This has been a theory that I developed on one of those long college nights, and I thought it was worth sharing. As the combination of alcohol, very little sleep and good company tends to do, my and my roommate's humor threshold was becoming surprisingly low, and thus he came up with the idea of watching youtube videos of people falling down. I have to say that I will never understand why we as human beings find it to viscerally funny, and maybe a theory to that effect will enlighten me on another night. However, on this night my roommate made the keen observation that babies and extremely drunk people fall in a very similar, very distinct way.
As soon as he said it I understood exactly what he meant. Usually when people fall down we expect some kind of reaction: arms flaying, bracing the fall, even a wince would suffice. When it comes to these two groups of people on the other hand there is none of that, they just seemingly fall in slow motion and just let it happen, and therein lied my theory as to why.
Small babies still need to learn that falling down hurts. The normal reflex reaction to feeling yourself fall is something that is not yet instinctive in babies, because of the fact that they have not felt that pain enough for it to have developed. This is why I think they just fall over, it never occurs to them to brace the fall because they have yet to learn what falling means.
Alcohol, on the other hand, is known to slow the reflexes, and being seriously drunk slows them significantly. Because of this, my theory is that drunk people may know that falling hurts, but they don't realize they're falling until it's too late. By the time their brain has processed the information that it is falling so they should do something about it, they have already smacked the ground.