I'll answer with a little double talk: students are interested when something interests them. Do adults read materials they are not interestd in when they have a choice? If we have to do something we have to do then we do it, but we don't necessarily like it. When we have a choice, we do things we like.
Readings (I'm just using reading as an example; we could talk about anything) that are about something students are interested in, interest them. Readings about subjects students aren't interested in, don't interest them.
The question, then, may be: what kinds of things interest students? What kinds of things entertain them? We all want to be entertained.