I agree with your comments about the lore. There are places and times to discuss less prevalent space marine chapters with different philosophies, but for the most part- space marines are not human. Even their intelligence is elevated to superhuman levels. The reality of their purpose is ever present- to die in combat serving the emperor. This is nearly every astartes' fate. On missions like the one in the Secret Level episode with Titus and his mentor's squad, especially against chaos magic, the marine stoicism guards them against only the most powerful emotional influences of chaos. Magic is cheating- especially chaos magic by a sorcerer of the chaos god of magic, lol. Titus is an absolute beast for beating that warp garbage. Astartes know that they are freaks of nature, but they are also above human level all the same, so most of them seem to take a serious, warrior-monk sort of approach to their lives. They just realize what they are, and the necessity of their existence. It's not fun for most of them, but they do it anyway because humanity needs them to.