There is a difference between the kind of pride that God hates and the kind of pride we can feel about a job well done or the kind of pride we express over the accomplishment of loved ones. The kind of pride that arises from self-righteousness or vanity is sin, however, and God hates it because it is a hindrance to seeking Him.
Why is pride so sinful? Pride is giving ourselves the credit for something that God has accomplished. Pride is taking the glory that belongs to God alone and keeping it for ourselves. Pride is essentially self-worship. Anything we accomplish in this world would not have been possible were it not for God enabling and sustaining us. That is why we give God the glory—He alone deserves it.
