Carl Rogers is one of the fathers of humanistic psychology and big proponent of having an unconditional positive regard towards clients.
One of the reasons that this quote speaks to me is that it acknowledges the fluidity of human experience. In American culture we tend to view people as made up of a set of stable characteristics, but that is an illusion. We are constantly in flux, changing and growing and adapting. This fluidity speaks to the hope that I have for change and movement when working with folks. For example, just because you see yourself as someone who is poor at setting boundaries, this does not mean you NEVER set good boundaries with anyone, ever, in any situation. It means you may have a pattern of poor boundaries this has become the story you tell yourself about who you are and how you behave. And these stories we tell ourselves shape our future behavior and what we believe we are capable of. Well, you are capable of so much more than that story you tell yourself. When people go through trauma, they have a tendency to become less flexible and fluid in how they respond and adapt and that is why therapy can be such an important component to getting better and gaining greater flexibility. One of the first steps is simply being open to the idea that maybe you have more possibilities and ways of being in this world than you give yourself credit for.
I also love this quote as it recognizes and normalizes fluidity in all aspects of ourselves. This most definitely applies to gender and sexuality. We are never set in concrete with how we view ourselves in our gender or our sexuality. What it means to me to be a (straight, cis) female has changed and grown and evolved over time. This makes me no less of a woman now than I was then, and the same goes for folx in the trans and non-binary communities. It is completely natural and okay to change and evolve and grow in these areas. How you view yourself today does not invalidate how you saw yourself yesterday or last year. Each is valid, no matter your gender or sexuality.