Why is it that there are times the mind wants to be happy yet the body seems to not want to cooperate?

The mind that wants to be happy is probably the conscious, thinking mind whereas the body is being controlled, in your described situation, by the reactive/instinctive subconscious mind.

Do you mean body in a literal sense (your body feels sluggish/lethargic), or body as in you feel sad or down, but are wanting to or trying to feel happy instead?

I am happy in spite of any lack of cooperation.
Then there are the times when it does cooperate and that only doubles the appreciation.