Something you willingly work hard at without being paid for is a labor of love. You don't plan on selling any of those embroidered portraits of your dog — stitching them was a labor of love.
A hobby that you love doing is one kind of labor of love, and another kind is hard, unpaid work that benefits another person. Taking care of your elderly grandpa is a labor of love, and so is practicing your skateboard tricks over and over until you get them exactly right. This phrase originally comes from the Bible, in both Thessalonians and Hebrews: "For God is not unrighteous to forget your work and labor of love..."