Look at it this way. A perfect trip is just a list of nice pictures. But getting lost and finding that weird local diner, or missing your train and meeting those random people, that's the stuff that STICKS. You remember the stress and the laughs way more than another pretty sunset. Those mess-ups turn a vacation into a real story you actually feel.
Exactly... it's not that the log is only for mess-ups. It's that the mess-ups force you to write down the real stuff, the feelings you'd otherwise skip. You jot down the panic of that missed flight, not just "flew to Denver." Later, that panic is the whole story. The perfect days are just a nice backdrop for the actual plot, which is always you figuring things out.