The survivors will be dealing with a planet that is striking back with natural disasters as the nuclear reactors left over from the first nuclear apocalypse begin to melt down.
Clarke and Co. have only six months to literally save the world, and she may need all the diplomacy she can get.
The Clarke/Lexa relationship had political ramifications as well as romantic as both young women realized that the old ways of Grounder culture needed to change if everybody on the planet was going to survive in some semblance of peace.
Clarke began to see Grounders and her people from the Ark as part of one big group rather than many separate tribes thanks to her time with Lexa, and that state of mind will likely be instrumental to her actions in Season 4.