Why is Sans's room on fire?

The fire/lights are meant to add an air of mystery around a character who knows more than they let on. He's extremely powerful, knows about parallel timelines, and has ties to Gaster (a character who we still know very little about). Until you get the key to Sans' room, you will not be able to enter, and getting the key involves a process that most people probably wouldn't do naturally (though it is very simple). Since his room is inaccessible, people expect a some kind of fantastic secret to be behind the door, when in fact his room is rather plain (and a bit silly).

In other words, the fire is only there to get people to be curious and ask questions. It has no purpose.