Ogrork the Mighty said:
Not having a lifetime of hatred built up, perhaps?
I can definitely see why a ranger shouldn't be able to swap favoured enemies routinely.
Except that you can encounter an aberration for the first time in your life at level 4, then choose aberrations as a Favored Enemy a few days later when you reach level 5. There's no requirement for a Favored Enemy to be the result of "a lifetime of hatred", especially since you can now select your own race as a FE (unless you're implying that this would mean that you were secretly filled with self-loathing, rather than simply trained to fight your own kind).
Now, from a game balance point of view, the main balancing factor of the FE is that it is circumstantial--it doesn't come into play all of the time, or even most of the time. The Holy Grail for the ranger is to be able to pick the right FE for the campaign he is playing in, as it will easily bring him up to the level of other fighting classes. For example, if you know that you're about to embark on a multiple session quest against the drow, and you select Elf as your FE, you're going to kick ass. That's part of the name of the game with picking a FE in the first place, but for a Ranger normally, it's a game of short-term vs. long-term. You can pick the enemy that would be most helpful NOW, knowing that later in the campaign you might not face it, or you can try to guess where the campaign will be going and prepare in advance by picking a FE you are likely to face later, knowing that you will be rewarded with a higher damage bonus.
A ranger who could change his past FEs in advance of an upcoming adventure would be at a huge advantage over other fighting classes, because he could have his cake and eat it, too. He could choose the FE that was helpful now, and later, he could switch it to something else. It stops being a question of careful preparation and starts being closer to a floating bonus that can be applied as needed. There would be no reason NOT to swap it every few levels if you had any inkling of what was coming, unlike the other retraining rules.
For example, a ranger starts at level 1 with kobolds as his favored enemy. The DM runs them through a series of kobold caves, but by level 5, they haven't fought any in a while but the DM runs them through a lot of old crypts. So he retrains his original FE to be Undead, and picks his new FE to be Constructs. But by level 10, the crypt phase has given way to extraplanar adventures, so he retrains his first FE to be Outsiders (Evil), his second to be Outsider (Chaotic), and his third to be Outsiders (Lawful), just to cover all the bases. By level 15, a dragon hunting quest causes him to swap out his earliest FE for Dragons...you get the point.
As suggested, I think if any FE should be allowed to be retrained, it should only be the most recently gained one.