Haughty eyes and a proud heart, the lamp of the wicked, are sin! (Proverbs 21:4).
Why is pride such a big deal? From the way that the Proverbs talk about pride, you'd think that it was the end of the world. But why? It's something entirely internal, isn't it? It's one of those sins that doesn't necessarily have to involve other people (at least not at the extent to which others would be able to empiracly observe it). Ultimately, it seems to be much more related to attitudes than to actions -- so what's the big deal? Is it really a sin?
Proverbs 21:4 lays these questions unequivocally to rest. It's almost as if it's anticipating the questions. It's such a declarative statement. Just in case you were wondering, that "internal" stuff -- that attitude your carrying around -- is sin, whether it translates into external evidence and actions or not.
I think prideful people (including me) need to have such direct, declarative statements. Otherwise, we'd be too tempted to reason our way around it. We'd go on looking at the world through haughty eyes -- confident in our own ability to snuff out the sinful actions before they became manifest. And we'd totally ruin ourselves in the process. It seems to me that the biggest problem with haughty eyes and a proud heart is that they become a lamp to us, if we're not consciously corrected. We start to trust ourselves instead of God. We value our own opinion above that of God and others. And ultimately, we set ourselves up as idols and gods. Instead of letting God's Word be a lamp to our feet (Psalm 119:105), we trust ourselves to be the best judge of life's direction. Instead of allowing Jesus to be the light of the world (John 8:12), we consider ourselves to be illuminated. In the process, we take over God's rightful place of sovereignty.
This is the ultimate problem with pride. This is why it's a sin: it supplants God. And anything that supplants God is wrong, sinful, and destructive. Don't fool yourself. Haughty eyes and a proud heart, the lamp of the wicked, are sin!