Why do I always think about him or her?

It's perfectly normal to always be thinking about him or her, especially after a breakup, and particularly, after a long one. If you've invested your time and energy into this person, only to have the relationship end, it's natural for them to be on your mind. Try keeping busy and working on you, and soon, you'll be thinking of them less and less.

I know that when I think about someone a lot, it's most likely that I am attracted to him/her. Maybe you find this person attractive, maybe you find that you two have similarities and you want to know them better (and not necessarily in a romantic way). What are you feeling when you think of him/her? What do you think about exactly? Do you think this person thinks about you too? Analyze this situation and narrow it down to understand it more.

When i think about him or her it's usually associated with pain from the past or just remembering small or little things involving that specific person

The right hormones and neurons fired up. You had a sense of purpose. It felt nice and safe for change.

Because I care for them and maybe the are very important to me and the are special so I cannot stop thinking of them

It's called a heart ache the person that you cares and loved in your life is no longer around but the pain and memory of that person you cared about will remain

You always think about them because you probably have a crush on them. Tell them about it and see if they like you back.

It is because you have already get used to him, the feeling of having him in your life. That's why when he's gone, you felt like there is something missing, and you felt incomplete.

I do that sometimes. It's okay to miss people but don't let it linger too long. Find something to do so that it'll take off your mind from it.

When somebody is important to us, it is natural to have this person constantly on mind. It might mean, that you are in love with them, but not necessarily. Sometimes we do think a lot about people who we dislike, who we find weird etc. And that is perfectly normal. If you have just broken up with someone, it's natural to still have them on your mind. It takes time for the brain rewire. Don't fight the incoming thoughts. Just accept them, and let them go, like clouds in the sky. Sometimes all we need is time.