Running and feeling "off" on some days - why is that?

Y’know how it is when some days just getting started feels hard – legs feel heavy, it’s hard to settle into a breathing rhythm and you feel labored so much faster.

Then on other days, running just makes you feel so great from the get go – you feel loose and the breathing is easy.

Does anyone have any clue why our body feels that way? I could run the same route at the same pace under similar weather conditions and feel totally different at different times. And I’m not talking about aches or joint pain – just the ease or difficulty with breathing and overall sense of exertion.