My boys have always had these weird fixations with things. Almost a type of OCD level.
When Payton was just 3.5 he was OBSESSED with matching colors. He was so organized and would get upset if things didn't look "right". But his biggest obsession has been tags and sucking fingers. When he was just 1 yr old, he learned to rub silk tags on clothing and stuffed animals and it became a comfort for him. In time he added in sucking his fingers. We're still breaking him of that.
But his latest obsession is (finally) a good one! He reads. "Real" books. all day. He's reading level 2 readers with no problem. And he loves to read and figure out riddles. He's just like his Papas!
Maxwell didn't escape the OCD gene either. He's obsessed with his belly button. He gets upset if he can't reach it in the car because of his coat or shirt being in the way. He rubs it, taps it, pushes it...everything. And he has no decency. He'll do it anywhere! No matter who's looking.
Hopefully, someday SOON, both of my children will outgrow these compulsions they can't yet control. And then we'll probably having something more difficult and trying that they obsess over!