LotR -- Book 1 - Chapter 03 - Three Is Company

[Inziladun]

Originally Posted by Formendacil

It's noted earlier in the thread that the "true" relationship of lords and thegns, masters and bondsmen is a theme of the book, but the direct relationship of this to Pippin wasn't quite sewn up, and I think it's important here: both Pippin and Merry end up declaring fealty to great lords, but it's noteworthy that Pippin gets the far more difficult master: Merry serving Théoden is almost as idyllic as Sam serving Frodo; Pippin serving Denethor is not.

If the War of the Ring does serve, as Gandalf intimates, to "train" the hobbits of the Fellowship, it makes sense that Pippin might need the more difficult "lesson".
Merry was eight years older, but seems to me as mature as Frodo himself. Certainly, he handles the preparations for the journey from the Shire pretty well, and performs solidly in the Old Forest, at least until the Willow incident.

I think Pippin being placed in Minas Tirith, in the very eye of the storm, basically alone, forced him to grow up very quickly.