Why did Zhuge Liang distrust Wei Yan?

Wei Yan's personality is quite different from Zhuge Liang's. Wei Yan is "arrogant" and others are "shunned". Wei Yan belongs to such a great and majestic general that others feel "afraid" of him. Zhuge Liang, on the other hand, is "approachable" and Zhuge Liang's principle of employing people is "meritocracy", so Wei will naturally not be entrusted with an important task.