Great question. I explained "humility" in this post, so read this first. The "Got Questions" website has a post that answers your question succinctly. The end of the article states:
"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. “What do you have that you did not receive? And if you did receive it, why do you boast as though you did not?” (1 Corinthians 4:7). That is why we give God the glory—He alone deserves it."
Read the full article HERE.
Summarily, I believe it is okay to feel a sense of accomplishment ("pride") if you did a good job (e.g. serving at church or getting a good mark on a test), but the glory should ultimately go to God because He gave you the health, ability, wisdom and strength to do whatever we did well.
[Answered by Pastor HM, who wishes he was as buff as Gaston]