That line there might sound better with the word "heavily" instead of "heavy".
And, I believe that it's due to the fact all three other lines have 12 syllables, and this one following as eleven. Though, that might not quite be the reason, it's just one of the things I noticed. And, that can mess up the flow sometimes.
The seconds stanza actually had a great flow, even though the syllable count was completely off minus the two  counts. Which is why the last line in the stanza before might not flow right due to the syllable count.
"A mutter, a stutter
-A whispered out plea,
The look of 'I love you' that she'll never see."
I love that stanza, completely.
It's flawless, amazing and just perfect.
The entire poem is amazing, I see very few problems with it. Actually, heavy/heavily was the only thing I thought was off. And, that's just my opinion.
I love how you put in the whole " make him smile " line because so many people do that while in this kind of relationship. Be it a sibling, relative, or lover.
So, I think that this poem is easy to relate to and absolutely amazingly written.
The way you wrote it was simply wonderful, I really liked the rhyme sceme, you used alot of different synonyms so it was far from boring. The form of it was different so that contributed to the "not boring" factor.
keep up the work
p.s great job on the contest :P
Yes the thought into this poem is tremendous! It's fluency was just as amazing as the word choice, and I like the message you're putting out with this poem, as many others have said it could very well be related to. Therefore I believe you did a great job with this. Excellent!