Originally Posted by Mpray1983
Rhaeghar was a knight so it doesn't matter if your the lord of a great house since he was going to be overlord to all and still chose knighthood. Albeit because he read it in a book that he should pursue martial activities. I know why Ned is not a knight and that is because he keeps to the old ways and the aspect of religion/the seven plays a big part. Robert on the other hand being from the south and being fostered in The Vale who are known for their knights just has me curious.
That's a good point and probably should be addressed to GRRM in one of his forums. It's been years since I've touched the books, but I can't recall any explanation. gwsat's right about feudal society in general, however, even in our history, there were orders of knighthood that nobility were a part of, some of which were international (Order of the Garter, Templars, Hospitalers, etc). There was a path for selection, including training, ritual, vigil and swearing in. I'm sure there is something equivalent in Westeros. Younger sons of nobiility and branching off family tree members who accepted a soldier's life/career entered the knighthood. I doubt high ranking nobility ever had to...if they wanted a knight's title, it was probably given to them.
Logically Westeros should follow suit, but perhaps there was a reason for some high nobility to enter an order of knighthood. Jaime was a Kingsguard...that might be it. Rheagar as a prince, might have wanted to for some reason, but he would still have commanded thousands of knights of any banner before the revolution.