It kind of just means "Thanks sweetheart/sweetie/darling" etc.
I wouldn't say you have to fully LOVE someone to use the phrase, but you have to have some sort of liking for them. It's a very casual way of showing your appreciation for someone.
The word 'love' in this sense is a term of affection said to someone. It is commonly used in northern England when speaking to someone of the opposite sex, perhaps in a shop when they give you your change. In the Midlands they say 'duck' and in the South, 'dear'.
Saying "thank you my love" as in you mean a lot to them. So saying "thanks love" is basically saying thank you my love:)