Board Thread:General Discussion/@comment-27907368-20160412010659/@comment-26347196-20160517044630

I don't know if people has pointed this one out yet or not, but there's some other evidences to why Sans is actually not lazy:

First is evidenced in the fight against him. From his dialogue, it implies that he's a former scientist (and this is also evident in his workshop as well), and that he had been trying to work very hard on the anomalies of the time (aka when you load and reset), which then he ultimiately gives up.

Second is in neutral ending, specifically Papyrus ending. From the dialogues, Papyrus mentions Sans working hard at some paperworks (persumably blueprints? But this is speculation so scratch that), which means Sans starts working hard again as Papyrus is being put on stress.

I'm kinda too lazy to cite in specific sources, but you can look at the Neutral Ending page and Sans/In-Battle.