I've always seen Sam as the ultimate hero of the series, frankly.
A lot of good points have been made here, and I think the most important one has to do with loyalty. The bond that Frodo and Sam have, and all the fellowship, is a bond that you often see in comrades in arms. The sort of bond that makes people brothers and sisters through battle and war, willing to literally die for one another. It IS love, but love in the purest form, that you love someone so much you'd give up your life for them. There isn't a hint about any sort of sexual love between the characters, but it is love and devotion to keeping the other alive.
There is also the point raised about class, and it's an interesting one. Sam has sometimes been described as a dog to Frodo's master, that he is always there and ready to do what it takes and is loyal because Frodo has shown him kindness, a kindness that most masters don't have to show to their dogs for example. In the books it seems much more pronounced that Sam and Frodo are of different classes, so I think a more apt comparison is the noble versus servant analogy that has been floated and the fact that Frodo who seems of higher status would treat Sam as an equal creates such a fierce devotion that it becomes love for him, love for him to succeed and for him to somehow repay all the kindness.
Take care all.