Being in a band for over 40 years is a huge commitment. The people involved have to be dedicated to each other, and stand by one another even when things get bad. So in some respects, the very fact that Steven Tyler and Joe Perry have kept Aerosmith going that long is proof about how much they care for each other.
But their relationship goes deeper than that. They are as close as brothers (which is part of how the word bromance was created) with Tyler saying Perry's friendship is "easily the most important and complicated of his adult life."