Why do the bad guys in movies always put a self destruct button on their Doomsday machines?

Wouldn't it be like "Well, at some point someone's going to want to destroy this, let's make it as difficult as possible! Yeah! Just like 'Eff you! Now you have to work to destroy my machine!'" but instead it's like "Well, I'm going to put this giant button, clearly mark it SELF DESTRUCT and put the timer just long enough for the good guy to get to a safe distance."

Now, a real question, why do people make their own self destruct buttons?

My pseudo-aunt makes fun of me, calls me a "heartbreaker" because I tell her stuff. And this stuff usually has to do with girls. That like me. But I don't like them back, so instead of being like "Yeah, ok I'll move on" when I say "I like you as a friend, but not like that" or "You like me way more than I like you," or "No, I won't be your boyfriend" they keep trying, or they go crazy, or they unfriend me on facebook (still laughing at that), which doesn't help either of us. But, I mean, I can't blame them all that much. There's a certain female that I've liked since the 7th grade, which is weird because I usually get over people in a snap. I've given up, sure, but it's still kind of there. Like that little nagging in the back of your head, like "She's so nice to look at."

I'm just now realizing how creepy that phrase is...

But like I said, I've given up, I've accepted it, I know it'll never go farther. Which apparently isn't a factor to one certain girl that apparently really likes me.

I dunno.

And then there are the times you push the self destruct button yourself. Like not saying things you wish you had, but instead ignoring it when every synapse in your body is screaming DON'T DO THIS. YOU CAN STOP THIS.

We all have self destruct buttons. Some are just better guarded than others. Is your's the kind that have several armed guards, or the kind with just a wimpy little glass box?