This is a typical situation that you may also find outside photography. The Golden Rule here is: the answer should be as good as the question is. Never try to come up with the answer that is better than the question. Remember, you are not answering yourself, you are answering a person who knows as much as his/her question.
So if you are asked why do you buy a more expensive camera, tell them "It is a better camera". That's it!!!
If they are clever enough to come up with the next question such as "in what way it is better?", tell them in what way it is better.
If they ask "Is it worth it?" or "Does it really matter?" , say "It does to me, but not necessary to you".
That't it. Do not try to turn them into your religion, and all of a sudden it gets easy peasy.
Try this approach and you will immediately see how effective it is.
The common mistake people make in this kind of situation is, when asked they start talking to themselves, explaining it to themselves rather than to a person who asked.
PS If you are unable to answer your own question "Why the f***k did I buy this expensive body", then this is a completely different proposition.