If you look at the valving in a shock, there is only discs with holes to flow the fluid, so not much to wear out there. The piston seal can wear however and will "loosen" up the shock a bit. Temp as someone else pointed out can definitely make the shock oil viscosity change.

I suspect you're getting used to the ride and as summer heats up you're feeling the change. But who cares? That's the beauty of adjustable shocks. Crank in a click or two every week until it's too stiff and then back it off a bit.