This saying is used as a means to express a liking of something that generally would be 'too good to be true.' It is to be said in a non-deadpan manner, with optimistic tone. When said, emphasis should be on the word 'my.'
For an alternative usage of this saying, it could be something that you dislike or would rather someone not say even if its true, but you must change your tone accordingly.
Often times this saying can be started by using the word "listen."
Dave: Finally I get a couple days off of work, I have a beach house that I have been dying to go to! Would you like to come with me?
Laura: Don't play with MY emotions, I'll have my bags packed in 20 minutes.
Dave: I cant believe we didn't even get to try the cake, its already all gone!
Laura: Listen, don't play with MY emotions, they didnt even leave us a crumb.