The melting pot is a metaphor for a society of initially diverse people becoming more homogeneous from cultural standpoint. The first thing to come to my head is the US cities that often were called so because a lot of very different people from all around the globe arrived there and finally formed a new distinctive culture which took traits and features of many nations.
Thus the phrase should mean that someone's positive view on and attitude to that characteristic feature of the society which united very different people to create a new culture expressed their delight to cross boundaries and embrace diversity of many nations and cultures.
Refer to the respective Wikipedia article for more information as JonMark Perry has already suggested in a comment to your question.