For you it's a sin, but for me...

Do all things without grumbling or disputing, (Philippians 2:14, ESV)
I appreciated Tim Challies tongue-in-cheek statement on complaining:
I’m not a grumbler, a complainer, and it’s a good thing, too, because complaining is one of those sins that I find especially offensive. Jerry Bridges counts it as one of Evangelicalism’s “respectable sin,” one that falls under our collective radar. He is probably right. But I’m onto it. I can spot it blindfolded at a hundred yards.
You must know what it’s like to be around those people who always find something to grumble about. Good news comes in and they somehow manage to find the negative spin. Bad news comes and they nod knowingly because this is exactly what they expected to happen. When a leader makes a decision, it is the complainers who begin to murmur, who begin to talk to everyone else, to ask the leading questions. You see them huddled together in the church foyer, you see them whispering together in the cafeteria after the meeting. They’ve got an opinion about everyone and everything and feel justified in being heard far and wide. They are complainers. They are grumblers.
My understanding of complaining is that it typically manifests pride. There is a pride that assumes that you know what others do not, that they have ignored information you believe is obvious, that the church, the world, the business, the family would work so much better, go so much more smoothly, if the leaders stepped back and allowed you to take the reigns. You may not say it quite like that, but isn’t that at the heart of most complaining—that your way is the best way? Do it your way and it will succeed, do it another way and it will fail. That’s at the heart of most grumbling. I hate that sin.
Now, to be fair, I guess I occasionally do some of this. I’ve got an opinion on most things and tend to believe that my way will work better than your way if you will just give it a chance. But that’s not pride, that’s selfless realism. After all, I’ve read lots of books and have experienced quite a bit of what life offers. Here’s the difference between me and you: I’m not complaining; I’m just humbly expressing what is true—that you should give me a hearing and then do things the way I suggest. Even though I may have an incomplete grasp of the facts, I definitely have the most important ones at my disposal and know beyond doubt that you’re wrong and I’m right. I’m not a complainer; I’m a servant! I’m not grumbling to others, I’m just sharing my thoughts and asking them to verify that it’s better than what you’ve suggested.
I’m no complainer! It’s just that I am especially gifted at seeing the facts, putting the pieces together, and charting a forward course. It’s a gift. When you do it you’re sinning, we all know that. But when I do it, I’m expressing love. It’s a spiritual gift in action. When you do it it’s proud grumbling; when I do it it’s humble service. That’s the difference between you and me. And that’s why I find your complaining so offensive. Yeah, that must be it.
Matthew 7:3-5