A denser living arrangement creates its own problems. Packing them tighter is just "stalling for time," "treading water," it's not a solution. If you jam 7 billion people into a suicidal mass, you still haven't answered the question of what to do when you reach 14 billion.
But never mind poverty, famine, and warfare. Overpopulation also has a more direct way of killing. Have you ever had the feeling that you just can't "get away from it all" anymore? Have you ever had the feeling that the Robinson-Crusoe vacation you planned for yourself last year was just an immensely expensive encounter with "guides" who smiled far too much? Have you ever tried to calculate the implications of the fact that the average square kilometer of God's green earth holds more than twice as many people as it did in 1950? Have you ever wondered why so many people talk to themselves nowadays, and why somebody once gave you a funny stare when you didn't know what "meds" were? You can circle the globe a thousand times, but you'll never find a "scenic vista" without a hamburger franchise behind you. You're unlikely to find a stretch of ocean beach, anywhere in the world, that doesn't have Styrofoam cups and frayed polypropylene among the seaweed. And don't dream of going back to live in your hometown, because it isn't the way it used to be: nowadays you have to lower your expectations and assume you have a quiet apartment if nobody is actually using a sledge hammer to do some renovations next door. You're permanently "burned out," not because you're "burning" anything, but because there are always too many people rubbing against you, to the point where you think your skin is going to come off. But, yes, there's an explanation: your glands are shriveling.