Am i too negative and too cautious?

my friend says so because i tend to point out what might go wrong with things. For example he bought a fake Rolex and wants to sell it as original on eBay, i told him that he might get in trouble for selling counterfeits. Another example, we got wasted and him and another friend wanted to tip over a coke machine. When i told him that the idea is stupid and pointless destruction he got all pissed and started mentioning how i am constantly negative. Am I?, isn't being a good friend is to look out for each other?