Funny how nobody thought he was a douchebag..
I'm wondering why we aren't talking about the fact that Leonard was a douchebag. Sure, he seems kind-hearted and is always there to help Michelle, leaving aside how much he hurts because he's friend-zoned..BUT he visits Michelle, listens to her, has sex with her while he's in a relationship with Sandra. Doesn't anyone care about that? She's the only one who has done nothing wrong in the entire movie. She's very caring, loving and on top of that, honest...something Leonard is not.
He kept hanging out with Sandra, not only because his parents pressured him to but because he was mad at Michelle. Notice how he mentions he has a girlfriend at the dinner with Michelle and her boyfriend, how each time he gets rejected by her, he then contacts Sandra. So here's what he is doing. He wants to pursue a relationship by Michelle but he knows it is difficult because she has a boyfriend so he sleeps with Sandra, keeps in touch with her. In the meantime, he still visits Michelle and hopes for a future with her while paying little to no attention to Sandra- just enough so that they don't drift apart- and when his dreams are shattered by Michelle, he goes bck to Sandra although he doesn't love her at all. Sure, he wouldn't want her to get hurt or see her die but he doesn't LOVE her and that's not fair to her because she's been nothing but wonderful to him.
He gave her the ring 5 minutes after he was dumped. He probably woke up the next day and regretted that move. Even if he didn't, let's say they got married and lived together until they got really old. Would Sandra have married him if she knew he led a double life and only chose her because he got dumped by his number one possible girlfriend?
I don't think so. You can't be a 'nice guy' if you are chasing after someone while making someone wait for you and having them think you like them just in case you don't end up with the one you actually like. Sandra had feelings, she wasn't a rebound or a plan B.