God has attached his blessing to a percentage (10 %) – or the tithe. Why? There may be more interesting reasons, but in short – We give 10% because God told us to. If he left it up to us to decide how much to give we would probably pick a much lower percentage. We would pick that lower percentage until we figured out someone gave more; then, our pride would want us to beat the other giver! Competitive giving produces wrong motives. Our stinginess and pride keeps us from giving with pure hearts. Therefore, God established a specific percentage to test our obedience. Everything we own is his, yet he asks for 10% from us as a test of obedience.
Aaron's Angle 
Having tithed for years, I can tell you that the old adage is true: 90% goes a lot further than 100%.
Aaron, I like that perspective. Stinginess and pride taken away in the tithe. And the amazing thing is is that a church that tithes as a whole has more than enough to see it’s vision through! I’d say God knew what He was doing.