Why do we fall?

So we can learn to pick ourselves up

Because if you don't like yourself or your own company you need someone else's love to love you so you can like yourself, and if you can't get that validation, you obsess fixate and imagine it because your brain will react as if it's real.

Love is the one thing that cannot be explained. Not even science can explain it either.
Relationships can be platonic, but I believe when people idealize a platonic relationship when they haven't found The One yet, they'll end up having sex more often than planned with The One. Not saying all, just saying it's a possibility. You can have a platonic relationship and only have sex when you want children. I believe that's the kind of relationship my Christian aunt and uncle has. They gave birth to two children and I had never heard them have sex on the regular. In my opinion, I don't know how they did it. Though at the end of the day, everyone's different.
Acting on impulse is one of the obvious signs of love. That can be friendship love, or romantic love. Being impulsive plays a part in both.

We hope that we can change the outcome of things. That’s the answer to all your questions.

We hope they change. We hope they will love us back. We hope for that fairy tale ending.

Sometimes it can happen, but often it does not. We take that risk with our feelings.

Don't try to dig too deep into yourself you'll get stuck up your own ass trust me over complicating things only dose that

It's partly evolutionary and enviromental. That is to say how we are brought up through primary and secondary socialization and by inherent characteristics from human ancestry.
Past relationships/friendships also contribute to present and future relationships/friendships. Our schemas will rationalize how we interpret people and ultimately, our brain chemistry will react only to those that we are attracted to.