Why are there so many pairs? Well my answer would have to be something simple and short, unlike everyone elses.
If you have 2 people, they can both use their qualities to help each other. Ex. Frodo is strong enough (physically, mentally, and emotionally) to bear the Ring, but anyone would give in to its power sooner or later. So Frodo is the Ringbearer, and must carry the burden, but Sam is there to help him when things go wrong. In my opinion, the Ring could not have been destroyed without Sam. I`m not talking about Gollum, being caught in Shelob`s Lair, and putting on the Ring and attempting to leave instead of destroying it. Yes, Sam helped in all of these (except the latter, but I believe he would have done something about it), but lets say none of this happened. Frodo makes it to Mordor without too much trouble. Just one problem. Sam isn`t with him. If Mordor were empty of everything, except Sauron and Mount Doom, I do not think he would be able to destroy the Ring. If Sam were there he would give Frodo hope, but most of all, he would be company to Frodo. Just being there, I think would make a big difference. Now put it all back together. Sam gives this hope and company, thus helping Frodo with his burden.
I`m having problems wording things today. I hope I made sense.
Nimrodel