The reason that Sans has to give a bad time is because of the Bone Daddies.

They want to do horrible things to sans, they will force him into marriage, they secretly draw him but in a much naster way.

They roam a tag on tumblr known as Undertail, poor Sans is tramaized, all he wants is to be comfortable, not an object for someon else's happiness.

Please save him from the naughty Bone Daddies, all he wants is a blanket, a good T.V show, knowing that Chara isn't awake within Frisk, and his ketcup.

And finally knowing that his brother Payrus is safe and sound.