Master Samwise can read

[Firefoot]

I think part of it is that Sam is a more "simple" Hobbit than Frodo, Merry, and Pippin. He is more of the "average" Hobbit. Frodo is, obviously, the Ring-bearer, and he is also 'higher' and more Elvish. Sam is more mature than Merry and Pippin, who act younger, especially in FotR, and are more adventurous. You note that when they returned to the Shire Sam and Frodo went back to their normal way of dressing whereas Merry and Pippin

"cut a great dash in the Shire with their songs and their tales and their finery, and their wonderful parties. 'Lordly' folk called them, meaning nothing but good; for it warmed all hearts to see them go riding by with their mail-shirts so bright and their shields so splendid, laughing and singing songs of far away; and if they were now large and magnificent, they were unchanged otherwise, unless they were indeed more fairspoken and more jovial and full of merriment than ever before."

(Talk about run-on sentences!) Sam has more plain-Hobbit sense. His wish in Mordor is for regular day-light and fresh water. He isn't stupid; his values are just in different places. Back in the Shire, he is counted as different for liking tales of Elves and Dragons. Sam isn't stupid; he's just different. He actually has a larger grasp of other things outside the Shire than most Hobbits do, but from the other hobbit's perspective this is queer, and seen as chasing after things that have nothing to do with him.

I'll just end with this quote from A Short Cut to Mushrooms. I think it is a good reflection of who Sam really is:

"They [Elves] seem a bit above my likes and dislikes, so to speak," answered Sam slowly. "It don't seem to matter what I think about them. They are quite different from what I expected - so old and young, and so gay and sad, as it were."