I always struggle with creating portfolio items. Partially due to the my own reserved personality. I struggle to express in a favorable way what I did in any given assignment or in my portfolio. It's not an issue of understanding the importance, I just find it difficult to both explain why I made the decisions that I did, and to take credit for the work I've done. I never want to feel like I'm boasting and I typically get nervous when someone else is evaluating my work. I guess most of us feel that way at some point.
Regardless, it is always a good exercise to think back about a project and see both the positives aspects, and the areas in which you can improve and I think that particularly with portfolio pieces recognizing both of those areas is important, even if not stated for everyone else to see.