(18f) my bf is obsessed with my shoulders and I’m wondering why.

My bf (21) has a total obsession with my shoulders, and it seems to be getting stronger with time. He loves touching them, and kissing them. He loves it when I wear anything that shows them off, particularly off-the-shoulder crop tops, or anything with spaghetti straps, or strapless. As a special treat sometimes I dance for him with oil on my body, and he always likes to make sure my shoulders are drenched in oil and looking shiny. This drives him crazy, to the point that he can very quickly cum over seeing that. I've never heard of anyone having a shoulder fetish before. I'm more than happy to indulge him, I'm just wondering if this is a fairly common thing for guys that I just haven't heard of?