Sorry for your loss. Pets big or small give love and happiness. They aren't looking for ways to cheat you or browbeat you like some humans (looking at you WT) they just see you as the center of their universe (ok maybe not cats) and love you.
When they get older, you think if the time comes and they are suffering, you'll just have to put them to sleep (logically) But when the time comes, you waver. Never easy no matter how much you know it's the right thing to do.