When I was younger I had this idea called the unicorn theory.

Its core tenant was that when we fall in love with our significant other, we actually fall in love with a unicorn. We fall in love with the instance of a being we could not fathom living our lives without. But that being is as much an imaginary creature as a unicorn because people change. People learn and discover new things that alters the way they interact with the world.

The unicorn theory is the reason so many people divorce or fall out of love. Its the reason why people fight in a relationship. The words, "since when did you..." fill in the blank is when love begins to deteriorate.

The excerpt you posted depicts true love. True love is that willful sacrifice an individual makes to go on that journey of change with someone else.