Board Thread:General Discussion/@comment-37345491-20190406164451/@comment-31147727-20190603171804

While Sans doesn't develop much as a character throughout the game, I think that's largely because most of Sans' character development happened PRIOR to your arrival. He discovered that his world can reset without any warning at the whim of a single person, and fell into a state of apathy PRIOR to you falling down the mountain. He made a promise with Toriel to look after any human that exits the ruins PRIOR to your arrival.

On the subject of the promise, I think Sans' statement that if it wasn't for the promise you'd "be dead where you stand" implies that he may have a prejudice against humans, which would make that promise a big step in character development.