Why would pride be one of the seven deadly sins?
When we study pride, we will realize that pride is the queen of all vices and sins. There is nothing that compares to pride - not even close. In fact, pride rules them all. For example, a person can control fear of man by an appeal to pride - "be a man," someone might say. A person can also be forced to overcome sloth by an appeal to pride; someone might say, "real men are busy." In the end, pride, another vice would be the motivation.
As you can tell from these two examples, the pride rules all the other vices.
There is something even more profound about pride though; it is always competitive. For example, it does not matter that someone has a lot of money; he or she wants more than another person. To use another example, it does not matter how beautiful a person is. As long as there is a person who is more beautiful, a person's pride will not be able to take it. The competitive element in pride corrodes community.
For all of these reasons, pride is the worst and this is why Bible says:
"God opposes the proud, but gives grace to the humble."
Moreover, if there is a Christian virtue, it is the opposite of pride, humility.
Pride can be defined as an excessive belief in our own abilities and our own power. This is a deadly sin because it means that we think that we ourselves are powerful and that we can get by without God.
Pride is what happens when human beings forget that they are insignificant in comparison with God and that they are dependent on God for their very existence. When they forget these things, they start to think that the good things that happen to them come about because of their own qualities. They think, for example, that they have come to have a lot of money or to be healthy because of their own hard work and talents. They forget that this is all God's work.
Pride is a deadly sin, then, because it leads people to think that they are capable of doing without God.