First of all, Opehlia herself describes how Hamlet had courted (dated) her, with all honorable intentions, and with every indication that he was completely in love with her. So, here she is, thinking that Hamlet and her are an item, are on the path to marriage, and that he loves her. Then, he starts acting strangely. He ignores her, and pretty much drops the relationship. The most he does is look at her longingly, and sigh. So, that's frustrating. She's ready to pass off as grief for his father's death, but then, the really cruel part comes in. She gives him back all the love trinkets that he had given her, saying, since they were over, he might as well have them back. At this point he shuns her, tells her to never marry, to go to a nunnery so that she cannot produce children, ever, tells her that he never loved her, and walks away. Imagine the guy or girl that you were practically engaged to, basically spitting in your face, telling you it was all a lie, and that you are so undesirable that you should never get married or have kids. Pretty harsh, if you ask me. I hope that helps a bit; good luck!
I think that it was all part of his scheme to look mad. The play makes it clear that they were in love; then Hamlet begins treating Ophelia in ways that make no sense, ways that may suggest insanity to those around him. The only problem I see with this interpretation is that it would have been simple for him to take Ophelia into his confidence so that he could achieve the effect that he wanted without causing her the pain that eventually led to her death. We know that he never stopped loving her. At her burial, Hamlet says:
lov'd Ophelia: forty thousand brothers
Could not, with all their quantity of love,
Make up my sum.
This might argue that his insanity was more than feigned, but his frequent returns to "sanity" lead me to believe that this is not the case. There is plenty of evidence for both views, and if you look at the criticism, many arguments on way or the other.