I like your ending phrase of "too happy." It manages to create several emotions at once and leave the reader with them.
The ambiguity of this is also nice. It makes you wonder what this person is hiding behind their smile. A lot of thought provoked from so few words.
My only nitpick about this is the rhyme. I guess it works okay for the first two lines, but by the third, the need for the rhyme automatically made me read it "hap-PY" (with the stress on the second syllable).
I really liked this. I loved how simply it was portrayed... it makes it feel more honest and real... if that makes sense. The emotion is subtly, but well-conveyed. Kudos.
Only thing I can think to criticise is that, without any proper syntax, I found it a bit hard to follow. But that's onl a minor thing :)