Perhaps when a friend does something cool that we aren't doing ourselves, we need to justify why we aren't doing the same thing. This happens a lot with activities: If I don't want to do something, I'll usually say that I'm not interested, but many people will say that they're horrible at the activity in question. Of course, every person who claims that they would suck an activity would probably not suck after doing it for a year. Everyone had to start somewhere. So is it just more socially acceptable than to say that you would suck at something than to say that you're not interested? Or do we feel like our friends who do cool things are looking at us and thinking, "How come you don't do anything this cool?"
Maybe some of these self put-downs stem from actual jealousy or feelings of inferiority. But what's interesting about jealousy is that, like the put-downs, saying "I'm so jealous!" has become another way of saying, "I'm so happy for you!" Not that it's this way all the time - I'm sure people express jealousy as a compliment so that they don't express it in a negative way. But I have often said, "I'm so jealous," when I didn't feel that way at all. It stopped for me when my college classmates would talk about their cool study abroad programs - I would be about to say, "I'm so jealous," when I realized I couldn't say that when I didn't even apply to study abroad myself. I'm sure a lot of people are jealous and really do want to jump into their friends' suitcases, but I wonder if being jealous has just become a compliment, the same as saying "I could never do that."
The point here is, we don't have to put ourselves down to put other people up, or say we're jealous to let people know that we're happy for them. Speaking that way could actually start to make us feel bad about ourselves. Because in truth, your tuba-playing friend isn't looking down on you wondering why you don't excel at the tuba - they're probably thinking that you're a great listener or that you always make them laugh or anything else that makes you you.