It's simply not acceptable to have to stand on a bus. Ever. It should be treated as some sort of breakdown/failure condition. If it happens on more than a few % of journeys, the public transport authority should be putting on more buses.
Otherwise people are just going to take their car because they actually have control over their own comfort.
With trains it's a little harder due to physical constraints on the network, but with buses it's inexcusable to not just have more capacity, particularly at peak time (London buses get silly in the morning peak, for example).
edit: Replies here are completely missing the point. A car has comfortable seats whenever you want them. It's honestly a comical joke to have to explain "why I don't want to stand up". The obvious answer is because I don't have to in my car.
Public transport can be, and should be as comfortable as that. It can't be as private, sure, but there's no need for it to feel like cattle class.
I'd gladly pay twice as much for it to be better. It easily could be; in London for example if everyone gave up their cars and put the money towards public transport we could double or triple the number of buses.