They both started out as earnest and fairly dorky young men (see Wes in s3 of BtVS and the flashbacks to Spike's mortal self, William) and have had to change their mannerisms to survive the cruelty of the world. A series of painful rejections and at times actual abuse - Spike by Cecily, his peers, Dru, Angel, and Buffy, and Wesley by his father, the Council of Watchers, and his AI friends, most importantly Angel, with whom he was very close - have caused them to create defenses to try to hide their tender hearts, which have been bruised and broken time and time again.

Wow, I'd never thought of that parallel between them. It's true that early Wes and William are very much alike, and Wes's AtS S3 transformation is quite similar to William's self-transformation into Spike. Very cool...