Feeling jealous of boyfriend and roommate

Sometimes I notice my boyfriend and our female roommate talking. I notice how much she laughs and giggles at what he says, when I don’t think what he said was necessarily funny. Over the weekend, I noticed myself getting annoyed at it happening again and also felt jealous. Part of me thinks she has a crush on him and I get annoyed by that. I always see her face light up when he comes home from work, but with me, I don’t see that same reaction from her, so of course, I’m finding evidence to support that she has a crush on him. He always tells me he’s not doing anything on purpose. He says he loves me and all that. We’ve been together for 7 years.

Why do I get jealous over something so innocent and trivial? Is it because I don’t trust him and believe him when he says all those things to reassure me? Or is it because deep down I don’t believe he really loves me?