Emma doesn't like Robin. It's that simple.
Since the moment Emma found out about Robin Hood, she felt anger, jealously, and, hatred, towards the thief. She should have been the one with Regina, not...him. She was the one who saved Regina countless times, and the one who would always be there for her. Not...him. Emma was secretly smirking to herself when she was in Neal's old apartment, and Robin had just confessed to knocking up Marian/Zelena, who was Regina's sister. It wasn't the fact that she was happy Regina's heart would be broken again, it was the fact that Regina wouldn't forgive Robin after that. Except she did.
Emma tried everything to make Regina realize that she was her soul mate. Why couldn't she see that? She even tried to make Regina jealous by throwing her self at Hook and practically hunching him when ever Regina came around, okay, maybe that was a stupid plan. But she wasn't going to give up. She was Regina's true love. And Regina would see that soon enough.