Why is political criticism seen as a personal insult?

This graph from Alex Massie's latest post over at The Spectator says a great deal about the current state of British politics:

At heart, the measure of how likely people are to see criticism of the political party they support as a personal attack on themselves is a measure of the sheer tribalism of their supporters, the degree to which they're political loyalties stem entirely from how they see themselves. As can be seen, no party is free of it, but (with the exception of the mildly Welsh Nationalist Plaid Cymru party) the more fringe the party is in nature, and the more extreme its policies are, the more tribal their supporters are likely to be.

Most prominent on this graph are the Scottish Nationalists, whose grass-roots supporter's ability to see any criticism, however substantiated by fact, as an insult to which they are entitled to respond with insults, will be familiar to anyone who has attempted to follow the development of "The 45" on twitter, and their hounding and jeering of journalists insolent enough to report unfavourable news about their leader.  We see similar behaviour from the self-proclaimed "People's Army" of the UK Independence Party, a nationalist movement in all but name. Whilst the Greens are not a nationalist party, that their support stems much more from their supporter's self-image than from any policies in their manifesto is hardly surprising when you consider that the Green Party's manifesto is in large part an un-costed shopping-list of things it is trendy to be seen to believe in.

To anyone those familiar with Chinese affairs, the phenomenon of people defending politicians who do the indefensible, and responding to criticism with angry invective purely because they have been taught to uncritically identify themselves with those politicians is nothing new. To see it become such a mainstay of British poltics is disappointing, however.

Happily, the likely failure of all of the fringe parties to make any great impact after the next election, and the compromises that will have to be made if they wish to make an impact, are likely to result eventually in a more realistic outlook amongst their supporters. For the time being, though, the supporters of the fringe parties can still engage in magical thinking whereby all that need happen is for Scotland/Wales to become independent, or the UK to leave the EU, or for 'neoliberalism' (whatever that is) to be brought to an end, and all their problems can be solved.