Normally in the early stages of meeting someone, if there is an initial interest and attraction to them, you may find yourself thinking about them all the time. Even if these thoughts are not all positive, it is likely that you are thinking about them a lot because you like them and your mind is trying to weigh up if this person is worth concentrating anymore effort and time on them or not.
It is likely that you are in the first stages of a relationship and to begin with this can feel almost like infatuation or borderline obsession. With current technology it is also likely that you can find out a lot of information about the person you are thinking about by looking on Twitter and Facebook to see if they are single and what their likes and dislikes are. It can feel like you are in love with someone, even if they do not love you back, although in this instance, it would normally be known as a crush.
What does it mean if i cant stop thinking about someone I've known for years?
If you have been friends with someone for a long time and you find that you are increasingly thinking about them all the time, it is likely that, for you, the friendship has grown into something more and you would like the relationship to develop. Often, long term friends are scared that taking it any further would ruin their great friendship, although, many people would say that this is what makes the best long term relationships. Ones that are based on friendship, things in common and trust should last a lot longer than a relationship based purely on physical attraction.