But why can't the fall asleep by themselves? I understand it's something they need to learn how to do, if you look at animals they don't have those problems, if we were to live in nature how is beneficial for us?
I don't really know the answer but I think in some way it probably relates to how much the human brain develops in the first 2 years. Their brains will never again expand at the same rate they are now, they are just learning so many new things about their world and how things work that when it comes to sleep in can be hard to reconcile all that new knowledge and just switch off for the day. BUZZING TOO MUCH TO SLEEP!!!!

What you do find is that some babies are just more happy to sleep than others, and some are looking for more help to get to sleep that others. I have a DD1 (5yo) who wants you there close as possible until she drifts off and a DD2 (3yo) who gets over stimulated with people too close when she is trying to sleep and needs her own space. It is a bit of a lottery what kind of sleeper you get, but I think whatever prize you get in that lottery there is a lot to be said for accepting the hand you've been dealt and not expending lots of energy trying to change it.

From what you've described and your babies age, getting over tired may be a possibility. Tired signs can be tricky especially early on. Seriously every mother has missed tired signs at some stage and ended up with an overtired cranky mess that doesn't know what to do with themselves, it's just comes with the territory! Some babies are more clear with their tired signs than others so it takes longer to work the less obvious ones out - DD2 I'm talking about you

I remember one piece of advice some one gave to me that I've always remembered is that that once they do the obvious tired signs (like yawning or getting agitated) it is already too late, you've missed the window. The less obvious signs are the ones that indicate the window of 'sleepy but not yet overtired and cranky' and the ones to look out for. These can be things like movements getting more jerky or eyes seeming to turn in more or wanting to stroke a particular surface/texture. It can be pretty cryptic or very subtle, maybe try googling tired signs and see what others have found and see if any ring a bell for you.

At any rate, I think 3 months is a pretty difficult time for you and him, it is still really early days and you are both learning so much about each other. I think at 3 months you are still in my 'do what ever you can to get through the day' grace period so cut yourself some slack! You are doing a good job, listen to your instinct and don't worry about what a baby 'should' be doing at x age.