I think I would be pretty reluctant to say should to someone else, as in 'you should'.

But for myself, I think it just represents my standard for myself. I should tell the truth because I believe it's the right thing for me to do. I should exercise and take care of myself. If i don't do what I 'should' I don't beat myself up for falling short of my standard. Just because I should do something doesn't mean I will. It just means that in theory, I want to.