Telling someone "you should ___" is a way of giving advice. You use the word "should" when you don't think the person will mind you giving them advice. For example, if you think that your friend is a talented artist but hasn't tried to do art professionally, you can say this:
You should try to do a show at an art gallery.
On the other hand, if you want to give someone advice that they're not expecting and might not like, you can use an expression like these:
You know you're supposed to finish that by Tuesday, right?
Um, you might want to clean up your desk a little.