I get your point and I believe that quite sucks.

There might be a number of reasons why they do that. Maybe it's just a reason they give for rejection while the real reason is somewhat uncomfortable to say. Maybe they are afraid that you will burn out (and that's legitimate concern, take a break!). Maybe someone enthusiastic just gives a better feeling, vibe, something not very conscious. Because while you are with someone energetic, optimistic, you suddenly become like him. When you are with someone exhausted, you might start to feel the same.

I get your pain, I respect your hard work, but I'd suggest to put yourself in their shoes - maybe there's something you haven't noticed? :)