Wonder why?

I went to see The Passion and I thought it was really beautiful, but then I started to think that dying on a cross seems like a really funny thing for a God to do.

We (Christians) always say that it was necessary for the forgiveness of sins, but if this is the case then why? Why would God have to demonstratively show his love for us. Couldn't he have done it any other way? Why did he have to suffer phisically for sin, when sin is usually an emotional or mental thing anyway. The more I think about it the more odd it seems to me. Maybe someone could help me understand the necessity of Jesus dying on a cross. Please don't say just because it fufilled prophecy, because God could have created different prophecy to begin with. Cors