A couple thoughts on why I oppose tithing as a requirement for Christians. 

Tithing isn’t what you’ve been told it is. The beneficiaries of tithing (the organization of the church) will quote Leviticus 27:30 and say “see, you need to give 10%.” But, that’s not the whole story. We find in Numbers 18 that the tithe is the Levites’ inheritance (you may recall the other tribes received land as their inheritance). Pastors aren’t Levites and the church isn’t the temple, so if you believed you had to tithe it wouldn’t even belong to your church. Unfortunately, most talk of tithing conveniently leaves out these passages. And what about Deuteronomy 14?  That says we use our tithes to fund the government, to pay for celebrations, and every third year we keep it to give to those in need.  
Second, in our struggle as Christians to understand our Jewish origins, we cherry pick the parts of the Law that suit our needs or that we think should still apply. What about wearing four tassels or forgiving debts every seven years? Read the early Church’s struggle with this in the Acts of the Apostles. It was hard for them to accept they could eat pork. It’s a difficult idea, but maybe we really are free to follow our Master as we understand it, using the Law as a guide (not a requirement) and listening to the internal witness of the Spirit. Imposing externals requirements is just instituting a new law.

My final reason for opposing tithing as a requirement is more philosophical: it excuses luxurious living and burdens those who struggle to get by. In terms of disposable income, 10% of your income is a lot when you make $50,000. It’s a drop in the bucket if you make $500,000.  If we instead look at giving as a method for being God’s hands and feet, and if you live a humble life, you will naturally give what’s leftover.  

Bottom line: I don’t have to give to prove my righteousness anymore; I give because I choose to.