Saria? Unhappy with being a sage?
I don't really think so.
The Kokiri, being childs forever, eternally, and having as much fun as a child can do, doesn't means Saria doesn't want to be a sage because she wants to go play wit the other kids.
In fact, when you get to the Kokiri Forest as an adult, the forest is infested with monsters, thus the Kokiris cannot even go play without being hurt, eaten, etc.
Second off, some Kokiris as Mido (who guards one of thosebig trunks and only lets people pass by proving him they are friends of Saria) go to the Lost woods and have almost no human interaction.
Third off, and finally, Saria senses danger in the Forest Temple, and she actually goes to the Forest Temple, not you, and, considering you never see her inside the temple, people can consider Saria dies in the Forest Temple, and she reencarnates in a sage after Link puts away the evil in the Forest Temple by beating/killing Phantom Ganon.
So no, I don't think by any means there is a valid justification as to why Saria would be unhappy with being a sage, rather than that she seems very happy by helping Link being the sage she is.
EDIT: Oh, I have just read baysiderulez comment that Saria can't die because you can ask her for help by playing Saria's song... well, who knows what happened to her after entering the temple? No one can know =S