I believe verse 28 gives the reason. Jesus tells us that we have little faith - "O you of little faith". We have little faith in God's future provision. I think whenever we have to give up on something, or give something away, we feel this pinch because we are unable to let go of what we have currently and see through the eyes of faith that what we God has in store for us for the future is far greater and better. Personally, I find it difficult to give, and that's because I'm selfish by nature. I want to keep and not share, because somehow it means that I will make a loss when I give something away or become poorer (relative). Perhaps, we also fail to see our worth in the eyes of God - to see that God sees us as His treasure, His precious child that He gave up His life for.
Luke 12 lists down reasons why we should not be anxious, and gives us promises too: