I'm not really sure why. But... do you stop loving someone just because they betray you? I don't think so. That's what makes the betrayal hurt so much - pain, frustration, anger... and I still loved her. I still do.
And Vin liked solitude. When you're alone, no one can betray you
I think given the choice between loving Mare - betrayal included - and never knowing her, I'd chose love. I risked, and I lost, but the risk was still worth it.