Why do we fall in love with certain people and why it is so hard to get over someone?

Well, it all has to do with you.

First, there are certain people you are destined to meet. And certain people your destined to fall in love with.
However this, from your perspective, is very fluid. You have free choice in every interaction.

The person you attract as your lover is a reflection of the parts of you that you wish to work on. And they will continue to be in your life until you have healed these aspects. (you can get rid of one and get another the same)

So, there is an energy attracting you to your destined lover and a connection of a mirror that is reflecting to yourself that which you deeply want to heal. And so, your body is all set to fall in love with these people. And you will stay attracted to these kind of people until you do your healing.

The way you get over someone is to heal that hole in your heart.
Whether its low self worth, insecurity, daddy issues... as soon as you heal this, actually almost as soon as you recognize it and start healing, that person will be gone from your heart and your life.

(the weird thing is that your new person, with the next thing to be healed, may be the same person)

Hope that gives you some insight.