Why won’t he have sex with me?

I’m 21, he’s 24 and we’ve been together almost 2 years (15th June). We’ve bought a house together and are completely in love, but he just doesn’t want sex and he can’t figure out why. His testosterone came back normal, so that’s been ruled out.

We’ve discussed it openly together and he just can’t work out why he’s not in the mood for it. Any help?