We haven't bought our first IP yet, but we're out looking, and we've copped the negative vibe thing from certain members of family. We just don't mention it much now to some of them. I know my wife's brother would not like to see us lose money, but he'd really enjoy rubbing our face in it if we failed, the classic "I told you so". His wife's brother (whom he doesn't particularly like) mentioned they were considering buying an investment property, and he openly told us how he hopes interest rates rise and "dickheads like him who don't know the first thing about property investment get burnt". I think that comment was also directed to us...
I would point out, however, that there is a difference between being criticised compared to what one might call reasonable or legitimate "what ifs". That playing devil's advocate can be useful to see if you understand what you are getting yourself in for, and what your contingency plans are if things aren't working out.
We've heard all the things like "What if you can't get a tenant", "What if they trash your property", "What if there's a property crash", "What if you lose your job", "What if interest rates rise" etc.
Actually, they're pretty good questions to be asking yourself. Motivational/insprirational books on investing (Rich Dad, Poor Dad, et al) may well have a tendency to push us towards acting in haste and without thought, our optimism overtakes our good sense, and a well-meaning family member/friend might well ask a question that you haven't considered.
There is a thread on this forum (going back at least six months I would think) where I was lamenting all the "criticism" my wife was giving me about property investing. I came to the realisation it wasn't criticism - they were valid concerns from her point of view. It took me a while to come to that realisation - prior to that I was thinking her criticism was "nonsense" or "a load of crap" or she was "holding me back".
The naysayers always throw up "what if" questions to rain on your parade. Most of the time they are making a big deal of something that is unlikely to happen. They'll then say "yeah, but what if it *did* happen". That's when it's worth having your contingency plan. There's an old saying that "If my Auntie had balls she'd be my Uncle".
Don't forget that there is such a thing as "constructive criticism".