Bruce Schneier: 

The "strategic" model of terrorism … posits that people resort to terrorism …  when they believe the political gains of terrorism minus the political costs are greater than if they engaged in some other, more peaceful form of protest. … Max Abrahms … argues that … seven tendencies are seen in terrorist organizations all over the world … [that] directly contradict the theory that terrorists are political maximizers … Abrahms has an alternative model to explain all this:  People turn to terrorism for social solidarity. He theorizes that people join terrorist organizations worldwide in order to be part of a community, much like the reason inner-city youths join gangs in the United States.

The evidence supports this.  Individual terrorists often have no prior involvement with a group’s political agenda, and often join multiple terrorist groups with incompatible platforms. Individuals who join terrorist groups are frequently not oppressed in any way, and often can’t describe the political goals of their organizations. People who join terrorist groups most often have friends or relatives who are members of the group, and the great majority of terrorist are socially isolated: unmarried young men or widowed women who weren’t working prior to joining.

HT to Larry D’anna.