A star has never been considered bad, in fact a star is quite amazing and we awe at the stars. It's not literal - you are not a star! But the expression is a way of expressing how the person is in awe of your positive deed. I use that expression a lot. I also like to say "you are an angel" or "you are a gem". They are very positive icons that make people happy.

Well is a way to express gratitude by complimenting you. Is not that strange thou! in spanish, i girl can tell you "erés un sol" (you are a sun) if you do a favor to her and she appreciate it!

It is a compliment. Like "You're the bomb" or "You're awesome". They are happy that you helped.

It is just an appreciation, or complement for your good deeds. It is like saying thank you in another way.