For certain days, you need a certain mug. Ask anyone. It's universal.
If you're drinking nice coffee, you need a pretty mug.
If you just need something to hold water, then that ugly one that looks like your Grandma's sweater will do fine.
On happy days perhaps you use that one that has a special significance.
The morning after a late night, you look for the one with the largest capacity.
Each type of day has it's own mug.
That being said, I have a magic mug that is the exception to every mug rule. I use it on any and every day of the week, and if it's dirty, I wash it. (even if there are other clean ones available! *gasp!)
On yucky mornings it makes me happy, on happy mornings it makes me happier. I love the unique shape of it and how it's proportioned. (I'm art snob, I know, but don't be a hater) and it has comforting shades of blue from the bottom up. I just love it.
We have a very special relationship, this mug and I. I received it as a graduation present from a dear dear friend of mine who also understands mug significance.
I suppose you're dying for a picture at this point, so I shall satisfy your desires:
When I pull it out of the cupboard I give it a sly (mental) wink at it's own irony, and proceed to fill it with coffee.
And now it is empty.
So I must go refill it.