I won't write a huge post, I'll get to the point:
Say that you have A project, you're halfway through and you get an awesome idea for B project. B project seems so cool, and A project is a mess right now, so it is very, very tempting to switch over and left A project unfinished.
Think of it as if you had to finish A project:
Inevitably your interest in A will drop somewhere along the road, so your motivation will switch from "A project is so cool I want to finish it!" to "B project is so cool, I must finish A project so I can start working on B!".
Thus you transform your motivation to drop A for B into motivation to finish it as soon as you can so you can start working on B.
Hopefully you'll manage to finish more projects before starting with the following ones that way.