Why is pride a sin?

There are different kinds of pride. There is earned pride, such as pride in a job well done. Rightful pride. And then there is unearned pride, such as pride in possessions or pride in ones social status. I suspect this second kind of pride, which really comes down to thinking you're better than everyone else, is the kind of pride Christianity condemns.

I second the answer by bone structure.  Here are some times when pride becomes a problem:

• I'm arguing with my husband and suddenly realize that he's right, but out of pride I won't admit it. I keep arguing and the arguments I use become less relevant to the issue and more about him as a person.
• I compare myself to others who don't give as much to their community, and I begin to think that I'm so much better than they are.  I even talk about what little they are doing when they could be doing so much more.  I might be condescending to them because, after all, I'm a much better person than they are.
• I love the Lord and try to live my life according to His will.  I see others struggling with sin and think about how much better I am than they are. That's spiritual pride.  Instead, I should mourn their situation and pray that they come to know and love God more.  And I should look at others who are more advanced spiritually, remember how far I still have to go, and pray that God will continue to open my eyes to His love and power.

Pride in the work I do is admirable.  I should take pride in my work.  If I don't, my work will not likely be that good.  Pride becomes a problem when we use it to put ourselves above others, because no matter how good we are, we're never as good as we could be. 

St. Francis of Assisi, on his deathbed, commented that he was the greatest sinner that ever lived.  He was a very holy man who loved God with all his heart.  Yet he never had pride about his love for God.  He realized that was a gift, not something he earned.  So pride can be misplaced when we give ourselves credit for something we didn't earn.

Hello Dr. Evil

The answer to your question and statement is in your name.

You and others have stated how a healthy pride exist. This is not the pride that God and the Bible is calling sin.

The pride that is sinful or evil is when you make a decision that you  don't need  the lessons of the Bible, Salvation, God, Jesus or Holy Spirit.

You choose to follow evil instead of ethics, morality or submit your mind to Godly rules. This is the pride of life, turning your back on God the father or Jesus  and choosing a relationship with evil.

This type of pride is sinful.

You have named yourself Dr. Evil, is this what you are proud of? God and evil are enemies.

A key agenda in the reborn Christian life is to grow in self-esteem.

(Jesus told us to love others as ourselves, and if we don't love ourselves, how can we properly love others?)

But it is perpetually difficult preventing one's self-respect from transforming itself into an elitism. Pride in one's association with others is good. Pride in one's dissociation with others is not good.

God wants us to be a "pleasing aroma" (Phil 4:18) — not "sniffy."

The kind of pride Christianity frowns upon is a self-centered type of pride.  Christians are to forget "self" and think always of the well-being of others.