nobody thanks you because:
1. you might be harsh with the next person so that person might to thank you
2. you might not behave well so they might not thank you.
3.it you help any one heart fully then every body is gone than you.
4.you have to behave well with each and every person you see or you meet at that point of view they cannot thank you but one or the other day they may thank you !!
hope so answer is useful and up to the mark