Reasons For Adopting

So I often hear people say that you should never adopt because you want to "save" a child (from foster care, from an orphanage, etc.) These people contend that you should only adopt because you want another child in your family. I can see where the anti-savers are coming from . . . parents who perceive themselves as having saved their kids will likely have unlikely/unhealthy expectations for gratitude and perfect behavior from their kids. On the other hand, it doesn't seem like an entirely bad motivation, either: the desire to help save a child from living in an institution, or from bouncing from foster home to foster home, or from aging out and being left to fend for him/herself. And adopting just because you want another child could likely create problems, too, right? Like if the child is challenging and requires a lot of work and effort on your part or disrupts your life, you may resent them for not making your dreams of a perfect family come true. So if you shouldn't adopt because you want to help a kid and you shouldn't adopt because you want a kid, what IS a good reason for adopting?