Ka-Ching!

Yesterday I learned about treasury bonds. I learned how China continues to buy tresaury bonds while we arent paying them back. It makes me wonder why China keeps on buying them. Essentially they are loaning their money to the United States and I can't understand why they do this... If I were to loan out money to someone, I wouldn't give them more money until the last debt was paid. I don't think it's so realistic for someone to pay a debt of millions of dollars. I don't know that much about the country and their money but from what I can see, it's not an ideal approach.