Obsess (verb)

1. To think about something constantly and to a degree that it affects one's daily life.
2. To preoccupy the thoughts of (someone) excessively.

Origin:

From latin obsessus, past participle of obsidere "to besiege".

Examples:

1. He obsessed over his ex-girlfriend for months after their break-up.
2. She obsessed over her weight and constantly checked her body in the mirror.
3. He became obsessed with collecting stamps.
4. She was obsessed with the idea of becoming famous.
5. The child became obsessed with the toy, playing with it all the time.