Why Lance is fire and Keith is water
A big mischaracterization of Lance is that he's not a fiery guy, when in fact his personality resembles fire in so many ways. He's hot-headed, far more than Keith, and that's something that's easily missed because Keith is such a reactive guy. Majority of the fights between Keith and Lance are started by Lance. Lance is also quick to always call out bullshit. He's very passionate about what he does and he always has something to say.
Lance has a temper and I think its so crucial to his character and his dynamic with Keith because without it, their relationship becomes a lot more mellow. Most of the time Keith is just standing like an NPC and Lance will get irritated by that and have something to say, hence causing a fight. Lance has the fire, always has, and it simply reflects on Keith.
I'd like to second this point by saying Keith's personality resembles water. Not that he's completely chill and passive, although he can be. But he's extremely turbulent and malleable. If Lance will start a fight he'll match the energy-he'll shape himself into a fighter. Keith has always been moving around from place to place and he's always had to mold himself into what's most suitable for survival, the same way water clings to a glass. He's got a lot of emotional conflicts within him. The fact that Keith's characterization is very similar to water is also alluded to in the fact that Lance is very drawn to both the ocean (water!) and Keith alike. He can't stop pursuing either.
Now I'm not saying that Lance and Keith are opposites when I call them fire and water because I don't think they're opposites at all. Both can grow/spread to great lengths, both carry a storm, and both can also be kind and gentle and necessary to live (ex: a fire provides warmth and water provides sustenance). Water can fuel fire, the same way Keith grows Lance's fire. Fire can also make water steam and boil, the same way Lance makes Keith react. If the water is greater than the fire, the fire will fizzle out; but if the fire is greater than the water, the water will evaporate. They'd have to be on equal levels for this cause-effect relationship to work. And Lance and Keith have always been on equal level (despite any internal insecurities). They always been neck to neck.
Keith and Lance had great characters that could've been explored more. Regardless of if you ship them or not, they had the potential to be very well written foil characters. Unfortunately that didn't happen :(.