Why does dormirse have a "se" at the end?
It doesn't need to because you don't "put YOURSELF" to sleep.
dormir = to sleep
dormirse = to fall asleep
"se" comes in case of reflexive verbs. In my understanding, since you have to put yourself to sleep, meaning no one else will make you sleep, append "se" to "dormitir" verb, making it to "dormirse".