My water heater is very close to my kitchen sink, yet I am only able to get very hot water when I have the hot water running at another source in the house. At this other source, the water is just warm. Why?

Possible deteriorated dip tube. Cold make up water comes in and is taken to bottom of tank by plastic dip tube. Hot water comes out the top. If dip tube disintegrates (and they do) cold water comes in at the top and moves across the hot water out. Hot water stay in the tank.