An Obsession, as previously stated, is an idea or thought that continually preoccupies or intrudes on a person's mind. So, to obsess about something (or someone) you have to really, really, really, really like that thing (or person). Take FIFA for example. If you like it, you enjoy playing the game, being able to sort the teams to become number one, going against other players, and like the game overall as mean of entertainment.
Now, if you were obsessed about FIFA, you would be worshipping the gameplay, be able to create a team that can destroy your friends (if you have any at this point) with only rubbish players, enjoy destroying and humiliating your opponents online, and deny any argument that the game is bad in anyway.
An obsession that is the most known is OCD, also known as Obsessive Compulsory Order. This obsession affects people of all ages. This occurs when a person gets caught in a cycle of obsessions and compulsions. These obsessions are intrusive thoughts or urges that trigger intensely distressing feelings. As an example, just because you like having guests taking off their shoes and leaving them by the door doesn't mean you have OCD. You just like the fact that your guests don't leave a trail of dirt around in your house.
I had a friend who once had to turn off and on the lights of his house 7 times when he entered his room because he was worried that if he didn't, his family and all of his loved ones would die in a horrible car crash. It didn't matter if he had just recently left the room and remembered he had forgotten something in his room, he would've already turned off and on the lights 7 times before leaving, and he would've turned off and on the lights 7 times before entering.