These are both fairly meaningless cliches, used to fill empty space and make a friendly response without saying anything much. The first - along with the equally vague related phrase 'You can't have too much of a good thing' - here means more or less 'Don't worry about turning up unexpectedly. I'm still pleased to see you' or 'I'm always pleased to see you, so it's really not a problem that you've turned up unexpectedly like this.' Here's another example: imagine that if you've invited your friend round and made a chocolate cake, and then she turns up bringing another chocolate cake. She looks embarrassed, and then you might say. 'Oh never mind, you can't get enough of/you can't have too much of a good thing, can you?' Or in other words, chocolate cakes are a nice thing, so having two of them isn't a problem. The psycholinguistic aspect works on two levels. You are assuring the friend who turned up unexpectedly, or the friend who brings the duplicate cake, that she hasn't committed a 'faux pas' by what she has done. You are also affirming your social bonds and sense of belonging to shared culture by using a familiar cliche which you have both heard and said many times before. The second comment works in a similar way. Maybe you are commiserating with a friend or a stranger about the uncertainties of life, or about the inevitability of death or poverty, or about the iniquities of the present government's fiscal policies. Whatever the reality of the situation might be, the underlying message is the same - 'You and I are in the same position. We both live in this world and this society, and we both agree on this issue.' What's more, both the speaker and the listener knows that this is a cliche. It's therefore a safe, harmless and non-contentious comment to make. Your profile says that you're interested in psychology, which is why I've gone a bit deeper, beyond the language, in this answer. I hope all this makes sense!

You can;t get enough of a good thing means if something or someone is good having more of it of being with them more is always a good thing to do. Like a good wine I can't get enough of it. About death and taxes, you got it perfect. You nailed it.