It doesn't decrease, it just changes (in some areas).

I still stare in wonder and my heart rate goes up every time I see her undress.
Anticipation becomes comfort, they become your safe place.
The 'I can't wait to see her again' became, 'I'd rather be at home with her than do anything else'.

Reciprocation is important, that's what keeps it alive. If your efforts are not returned, it's hard to maintain enthusiasm in the relationship and things will begin to ebb away.