It's much easier on the psyche (or the emotions, anyway) to say "I will do (something)" than to say "I should do/be doing (something)." Maybe it's just because "should" implies that there's no room for being weary or for a lack of time and energy. Or perhaps the effort of will in making a choice is stronger than the effort of will in complying with an outside standard.
I'm still mulling this over.