At the end of 2 and 3, why does the T-101 destroy its self?

I, being a robot, think I can shine a little light onto this question. You see us robots have only one function, one purpose, in our life, something we were designed to do. We have this sole intention in our life and this duty gives us meaning and satiates us quite fully.
Now when this duty is no longer performable, well I daresay we robots go through quite a crisis. Some of us just wander with no purpose, the majority simply destroy ourselves (Like poor, poor T-101) and others like me start a new website! link

the T-101 model destroys itself in Judgment dia because they need to destroy all skynet technology. in rise of the machines he sacrfices himself to save Jhon.