There are lots of reasons why terrorism happens. Suicide terrorism is discussed in depth in Robert Pape's excellent book Dying to Win: The Strategic Logic of Suicide Terrorism.
After compiling the most complete database of suicide terrorism over the past few decades, Pape identifies the most common source of suicide terrorism as a perceived imposition on a land, a perceived encroachment on one's land.
In the world we live in where many view themselves as part of a nation, part of a tribe, a part of reality is that borderlands will always produce conflict. Resources are finite, land included, so encroaching on another's land may understandably lead to violence.
That this encroachment is the most common cause of suicide terrorism is the well-studied, impartial observation of Pape's. With the entire country discussion terrorism, Pape's Dying to Win deserves a re-reading. In the days ahead, the media machine will throw talking points at you whether you read and watch the news or receive it through secondary or tertiary sources. Talking-points will mindlessly fill the discussions of virtually everyone you know.
No matter what comes out in the news about the Boston bombings, the fact remains – encroaching on the land of others is the key cause of suicide terrorism.
With American bases all over the world and our insistence in taking sides in tiny skirmishes like whether North Ossetia-Alania (a place we know nothing about) should have a different relationship with Chechens (a group of people we know nothing about) than Russians (a county we know next to nothing about), we must expect problems to come home. When we interfere in the business of others, we are engaging in the exact behavior known to generate suicide terror attacks. We must not be surprised when 1 plus 1 ends up equaling 2.