For many people, political activism for their causes takes on a religious role in their lives. It gives their lives meaning and makes them feel part of a larger whole in the way that religion does.
For other people, conformity to the values of their religion requires them to have certain political views, and to be active with them. Taking action displays their faith to themselves and others, because our values are shown by what we do.
Religious views become mixed up with political ones, to a greater or lesser extent. Religious values say that helping the poor is Good, that chastity is Good, or that being kind to other species is Good. Adherents then are prone to wanting those Good things put into law, or at least to have their government support their practice. And it is an affront to them for their government to fund things with which they disagree.
Like numbers brought to life, people see themselves as having a "right" or "left" sign. They belong to one side or the other, and think they have to conform to all of the beliefs associated with that side. The religious overtones for certain issues bring religious conformity to bear. It's peer pressure.
And I think without the desire to be seen positively by our peers, only the greedy and power hungry would be active.