In my view, this is appropriate query for any relationships. I guess everyone can attach the different meaning to it, for me it is not only about comfort, trust and reliability. Those are kind of "prerequisites". You should feel safe and at ease with the person, who is next to you for some time of your life.
For me, the "right" person is also the one, who inspires you to grow and to reach new levels. Person, who can show you the different possibilities in life. Self development is crucial during the entire life - hence it is critically important to be supported by the one who has the same vision, and who doesn't let you get drown in the routine.
I was lucky to meet such partner 18 years ago. Together with my wife we went through exciting and challenging periods of our lives and I know there are many interesting chapters for us ahead.