Luke 16:10-15; Luke 19:11-26; Matthew 25:14-29
Another reason we give is because we want to become people that God can trust with resources. When we refuse to tithe, we communicate to God that we don't trust Him with that part of our lives. Consequently, He is unable to trust us with more resources because we will inevitably end up spending it on ourselves, just like we already do with all of our provisions.
The reason we address this is because there are people who have said, "I'll tithe when I get a certain amount of money." As we're about to read in these passages, Jesus shows us that just simply isn't true.
When we're faithful with little, we'll be faithful with much. When we're dishonest or selfish in the little things (or the little that we have), we will be exactly the same regardless of how much money we have. And so not only do we teach our hearts to rely wholly and completely on God, but we also get the opportunity to communicate to God that we are able to be trusted with more!
Why do we tithe? We tithe so that God is able to trust us with more and so that more people can meet Jesus!