I think it's totally natural. However, since Max passed last week I think you realize that your urge to "replace" him is because of that loss. You also state the dog you're interested in looks just like Max, and I think that's telling. I would suggest giving yourself a little time first to grieve Max's passing before making any firm decisions. But it's entirely up to you. If you think that adopting this other dog will make you feel better, by all means do it, but make sure that in doing it you realize that it is not Max nor a clone of Max and will be his or her own unique self. Also realize that another senior dog means you aren't going to have a lot of time with them and will have to face their passing as well. It's hard enough to get through losing a pet, but losing more of them in relatively quick succession has to be a whole lot worse. A senior dog is also more likely to have health issues and thus vet bills and perhaps special needs in the not too distant future.
It's a lot to think about, but I think you would be well-served in giving yourself some time to think about it all before making a firm decision. Decisions made in haste and because of a need to heal a wound aren't always the best ones, but only you can know if getting this other dog would be right for you. That's another thing about decisions - we can never really be sure however much we consider everything if they'll turn out to be the right ones.
= new reply since forum marked as read
