I addressed how the Macedonians that Paul referred to in 2 Corinthians 8 were in great affliction with joy and give liberally out of their deep poverty. They understood that what they have ultimately belongs to God. We are just stewards of His resources. Sacrificial giving comes when we have completely surrendered ourselves to the Lord.
The ultimate example of giving is our Lord Jesus. He gave up the riches of Heaven, was born in an humble fashion, died a cruel death, all so we could be rich in Him. Because He gave His best, we should follow His example.
It is so easy to examine our income, expenses, and decided that we cannot afford to give to the Lord. We cannot afford to withhold from Him. He honors His promise of meeting our needs. This boils down to an issue of trust. Do we trust God enough to bless our giving and continue to supply our needs when we give sacrificially to His work or to a need around us?