thumb sucking question

my LO started thumb sucking a week ago (12 weeks now), and while i totally don’t mind, i’m curious about why he does it. he’s not a big crier, smiles and laughs a lot. but lately, it’s like he will cry out loud, just so he can suck his thumb. does that make any sense? so it’s not like he’s sucking his thumb to calm from something else bothering him. it’s like he is making a fuss so he can suck. or maybe something is bothering him? i have no idea. it’s probably just normal, ya? 🤷🏻‍♀️