ABE is not bitterness, its the opposite: having a laugh. Its exactly the same all over the world: if you have fought several wars against a country you don't tend to support it in football games. The past is in the past now when it comes to politics (for most at any rate), but it expresses itself in football games, especially since we don't have many chances outside of sport to differentiate ourselves from England on the world stage.
The main reason I don't support England though is because of the arrogance behind the assumption that 'I'll be supporting England then', and the shock on the faces of English and foreigners alike when I say no. You wouldn't say to a Costa Rican 'so you'll be supporting Mexico then?', or to a Korean 'so you'll be supporting Japan then?' just because their own team didn't qualify and they are neighbouring countries with similar cultures. In fact it's more than that: we have always been England's rivals on the sports field, and indeed on the battlefield: it would be like saying to an Arsenal fan 'so now Arsenal are out of the running for the FA cup, you'll be supporting Chelsea then?'.