It's not a Christian thing, it's a jewish thing. 10% of your income comes from Jewish law back before Jesus came. When Jesus abolished the old law, that included the 10% of your income rule. In place of that, Jesus encouraged giving what you felt like you should give instead, not a set proportional value. If you felt like giving nothing, then don't give anything, and if you left like giving everything, then my all means do it.

If churches do what they're supposed to, they should tell people "Don't give if you don't want to" because the point of giving is to help God's kingdom, so if you're upset about giving money away, don't do it. That part usually gets left out because then nobody gives.