Haven't read other responses, but the OP raises a topic which has always interested me.
I think you become a Democrat due to these basic psychopathologies:
1) Guilt. Your parents were better off than others, and you feel Guilty about it....and you try to sell "Compassion" but you don't want to donate your money to a charity---you want to tax us all especially the ones who had to struggle to make it, when you had it handed to you....which of course make you a Hypocrite.
2) Jealousy. Your parents were poor, and you have decided that others looked down on you, and you want revenge. The Jealous are the purest Socialists, and they are also Communists, Facists, or some other kind of totalitarian, but are too ignorant to realize it, because they don't understand that Socialism takes FORCE to strip a person of all he has earned so as to make him exactly equal to all the others who have sat on the porch and toked and fucked while waiting on their Gov't check. There is a natural inequality to Ability, and also to Motivation, but the Jealous don't accept that reality. They demand the Free Shit.
3) Unhappiness. For example, all really really butt-ugly women...women so ugly they couldn't get fucked in a Chinese prison with two bags of rice in their pockets--are Democrats. Unhappy people want change and the Democrats (Bolsheviks) want change---actually they want Power, but they call it change to recruit the Unhappy, and fool the next group.
4) Ignorance. If you are educated in a school system controlled by teachers unions in a rotting Northern city run by Democrats for half a century, then you are a Democrat because you know nothing...except possibly---how to call people in the Heartland ignorant rednecks.
5) Mentally Ill. This one of course is obvious, and requires no further explanation.
6) Illegal Aliens and the Dead. All of this group vote very reliably Democratic. The last is particularly galling. There are not many Republicans in Chicago, but there are some who vote consistently for Republicans all their lives, but when they die, they become reliable Democrats.
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Of course, there is overlap in these groups, but as a whole, this concoction of half-wits and whack-jobs make up almost half the population...and if they achieve a majority it will be the end of the greatest country thus far established on this planet...not perfect...but clearly Humanity's most successful experiment to date.