I find it mind boggling that people chop and change their political allegiances as if they were stood at Woolworth's Pick 'n' Mix Principles counter.
Teenagers may have allegiances that chop and change. Grown-ups generally don't, but many adults don't have strong party allegiances at all.
Back in the days when almost everyone in Britain who voted did so for one or other of the two main parties, the level of allegiance was higher. Perhaps the majority still have definite allegiances which change rarely (hence lots of safe seats), but fewer and fewer do. The choices many people make about which party to vote for are often not really allegiances. They are just guesses about what will turn out for the best, about who can be trusted, about who should be 'given a chance' (or not) and, yes, as you suggest, these choices can be a little bit like consumer choices.
I'm not sure what to make of left-wingers on Facebook, or wherever they are, who are spiteful towards some poor bloody mug who voted Tory. Perhaps if I met someone like that (one of the spiteful ones, I mean, not a Tory-voting mug) in real life, it would be easier to figure out what was going on. Maybe these people are just kids for whom politics and voting is some daft little game of identity that brings with it a tendency to be spiteful towards anyone in another gang. Maybe they are very sensitive people who are so distraught about the election of a Tory government that they are tearfully lashing out at any Tory voter within striking distance. Maybe they are nasty people whose compassion for others is at best highly selective and really just a thin cover for bossiness and bullying. Maybe they are just pissed or stoned and talking bollocks. Maybe they are joking to wind up people who are foolish enough to take their comments seriously. I don't know - I just don't come across anyone like that - and it's probably best for me not to think about it much.
What I do know is that people make crap choices about all sorts of things. Fucksake, people marry the wrong people, drive while pissed out of their heads, choose the wrong occupation, take the wrong drugs or gamble. Some people support Leyton Orient. Some people vote foolishly. In fact, many do. People seriously involved in politics will want to persuade those people to change their minds. Surely this is obvious, though also easier said than done.
Famously, Tony Blair and his chums understood that politics was about winning people over, including Tory voters. There is now a rather different Labour leader and I think he is also a firm believer in winning people over. To what extent he will succeed I don't know (probably not enough to win the next general election, but it's far too early to make predictions about 2020), but my impression of Corbyn is that he is a world away from the spiteful types.