Its not about anyone owing anything. Shenton is a Cas lad born and bred. Hes one of those that wanted to play for the club as a kid, the type that would say 'the money doesnt matter, i just wanna play for Cas'. Now hes an adult he has to consider his future so he's had to make a big decision. The 'wrench' he mentions is that heart vs head choice. Not that he feels obliged to Cas.
I agree though that hes done well but you gotta understand the desire to better himself. There are some players that get international call up just cos they play for a big club and its easier to get noticed. Shenny is one of those few players that Cas have brought up that stand out for their talent not their club.
The only downside is Cas dont get anything for losing one of the talents they have developed. He's out of contract at the end of the year so just moving on for free.