Dialog is tricky because you want it to sound natural, but nobody talks like book dialog. Real people say um, and pause, and repeat words and do all the things that would make reading awkward. What you want to do is give the appearance of real dialog, but take out all the awkward stuff (unless of course, you're writing someone who is awkward)
Realistic dialog will sound like people talking because what they talk about usually matters to the people talking (even if it's small talk) and it isn't story told through dialog. If you find yourself having a conversation that explains stuff about your world or book, odds are you're not crafting realistic dialog. (there are exceptions of course, but basic rule of thumb).
Characters should have reasons for saying what they do that come from the individualized motivations of that character. If they only speak because plot tells them to, it's probably going to feel fake.