DH has just been bitten by DS twice!!! Why would he do such a thing?

[Washersaurus]

We have had a terrible day with DS (21 mths) we're not sure if he is feeling a bit under the weather or is turning into some horrible little monster.

Truthfully, we were both keen to get him to bed tonight because we've had enough, so we were following our usual routine of tea, milk, bath, story, bed, when DS normally lovingly gives us hugs and kisses etc -when instead of hugging DH, he BIT him on the shoulder.
DS started crying immediately after doing it(and so did DH). We told him it was naughty and that it had made daddy sad..and then he went and did it again.

Why would he do such a thing?

becaise he is 21 months old.

there are many reasons, probably, and all of them will be complex, but this is what it boils down to.

Agree with Colditz, I think all children go through this stage. Just stay firm on the "not a nice thing to do, daddy is very upset etc" speeches and try not to make a big fuss or it becomes an attention strategy. Don't worry though - I think it's pretty normal. DD bit me last week on the inside of my arm when we were having a cuddle - I was a bit stunned as she hasn't done this before. Just told her it was not nice, she had hurt me and asked her to say sorry. She is 23 months. Her cousin is the same age and he is a frequent biter.
Can't offer any more help I'm afraid!