So you made a cost-benefit analysis and determined that the love you felt for the cat was more beneficial than the monetary cost of tending to his health. I made a cost-benefit analysis and determined that my time was worth more than the $5 it would take to replace a fish. You are willing to pay an increased cost to maintain an emotional connection, and I am not.

The point is that, as sole alluded to, the line is arbitrary. You value a fish but don't value a mosquito. As a vegetarian, you value a cow but do not value a field mouse/insect/etc. Hell, we can move further from ourselves into other kingdoms: you value the lives of animalia but not fungi. You value the life of eukaryotes but not prokaryotes. The more you zoom out your focus, the more arbitrary the line becomes. You're selecting the traits you value and protecting only those species that meet them. This is the major philosophical issue most people have with so-called animal-rights groups such as PETA: they have drawn the line at protecting what is convenient and visible, not what is logical and defensible.

I guess if there's a take-home message, it's that you are drawing an arbitrary line that is very different from where other people draw the line.