It's karma. What you appreciate appreciates !
When you express gratitude, you become part of divine acceptance. Gratitude is a sign that you feel joy in being part of the universe. The universe pays back this joyful embrace. It multiplies your blessings. What you appreciate appreciates !
Let's say you said "thank you" to your friend on receiving his gift in a most energetic, gleeful, and loving way. What would happen next ? The flow of gifts from your friend will keep continuing. This is the power of gratitude. What you appreciate appreciates !