Three simple words made easier

I tell my husband I love him everyday... Like most people I suppose do.  Tonight, I get in bed and we joke around for a bit and as things settle I tell him that I love him, to which he replies... Mm-hmmm. ??????

I slap at him and confess that it bothers me when he does that.  I confront him, infact, and tell him he has done that a lot lately.  He said,  mm-hmmm is just so much easier to say! Wth?  Then he started telling me he loved me over and over, reminding me of Sheldon from Big Bang theory knocking on penny's door.   Now it's just annoying!