puppy with diarrhea commonly occurs, actually. The problem with this, though - as compared to a common human sickness, like a cold - is that it could actually be very dangerous. So don't mistakenly connotate how commonly the dog gets it, with the potential danger of it. Just make sure you don't forget that.
So, what are some things that could behind the sickness that the puppy with diarrhea has? Let's find out... What I didn't mention was that, most commonly the bad stomach is only going on for a matter of days. If for more than that - if it hits the weeks - you should check out if there is anything else going on. In other words, MEASURE how long the puppy's had this loose poo. 
Nature works the same on us all. This means that, if you feel it would help you to understand how the puppy's stomach is working, then you could compare it to a human. So... if the baby's had some new type of foods, and shows an reaction towards this - even this a little more "serious" reaction - you have, in that, something that makes it understandable, and probably less dangerous, that a diarrhea's happening. "Ahh.. whatever. He's been eating this and that, which he never did before - so it's not that strange!". Think the same way for your puppy's bad stomach.
But you still want to be treating it. Because you care about the puppy's health. How it's treated comes from the diarrhea's level of severity. Home-treatment could work for some milder types. But if it's in liquid-form, and the dog cannot control it - your common sense tells you that, like if it happened to a human, something is wrong. If your gut tells you it's wrong, it's common sense that you would go to get it checked out.
A virus or a parasite could be the diarrhea's cause. Also, it could be infected - and getting a stool-sample, which is some poo, to check if it is, to the vetenerian would be smart thinking. This could be a worm-infection. The puppy's normally not checked for that before 3 weeks, but sometimes it can happen after two.

Hopefully, this has soothened your mind somewhat regarding your puppy with diarrhea - or will help someone you know with checking up if their dog has it.