- Your current job doesn't understand your genius (the fools!). They haven't treated you right, they don't know what they're doing, they're screwing everything up, they couldn't manage their way out of a paper bag, everybody admires you and hates them, and they don't deserve you.
- Your last job was a complete disaster, and they should have listened to you (the fools!). They were in a complete death spiral, they didn't recognize talent when they saw it, they took and took and took but didn't know how to give, and, let's be honest, they were all sloppy dressers.
But why? What if it's the truth?
Okay, let's take the point of view of the person on the other side of the table. They don't know you from Adam. (Or Eve.) They're trying to figure out a few things about you, including [A] do you have the skills to do the job and [B] will you be a good person to work with?
Most people -- probably including you -- want a supportive and productive office environment. A big part of that depends on management, such as those idiots who should have realized your perfection (the fools!). But a lot of the office environment... and here's the tricky part... depends on co-workers at the same level. The person across the interviewing table probably doesn't want to work with someone who screams at their spouse over the phone, who brow-beats their colleagues, or who's constantly complaining about management. And there's nothing like a 15-minute-long rant in the middle of an interview to make you look like someone who might not bring a lot of joy and sunshine into the office.
Venting is (usually) a good and healthy thing to do. But you should probably choose your audience a bit carefully. Friends might work. Former colleagues are a good bet. A spouse or partner can be good, but maybe that shouldn't be the main topic of conversation in your household. ("Hi, honey, I'm home, and you won't believe what that #&@#@* said to me....")
At least one interviewer will probably ask why you're looking for a job. I recommend that you think a bit about how to answer that simple question. You don't need to completely avoid negative topics ("I just love my job. Love it love it love it!!! I come to work every day with a smile! But please for the luvva Mike, rescue me!"), but maybe try to strike a balance.
Some things I've said in the past:
"When I first got to the company I really enjoyed myself, and I felt really good about the things we were working on. I still really like the people I work with, but now the company is doing things I don't feel good about. It's difficult to say this, but I think it's time to move on."
"I've had a great time, but it's a small place, and I think I need a larger company so I can grow more. I need new challenges."
"I really liked my last job. I mean, sure, I was frustrated sometimes, but most of the time I really enjoyed it. Then the company had some economic troubles and needed to downsize. 2/3rds of the company was laid off in a single day. It was kind of a shock. I was part of the 2/3rds. I wasn't happy about it, but it was nice to know that it wasn't personal."
Your own words and reasons will be a bit different, but you might want to have a friendly and thoughtful answer ready, and do your venting somewhere else. Or you might find that the company where you're interviewing doesn't offer you a job and doesn't give a clear reason. (The fools!)