Negative energy is indicative of negative, (at times even tangible), things.
Don't get me wrong, there have been plenty of times where I have walked into an "empty" space, and by empty I mean vacant of people, and have felt a distinct sense or vibe of negative energy, I cannot explain why this happens, or what triggers it, but the feeling is strong enough for me to leave that place and never come back.
However, usually when I feel negative energy it is in a social environment, where something is just off, where I am either surrounded by insidiously angry/hateful people or by a disjointed group where some type of fight will emerge.
Being in a positive environment is, as I stated earlier, just awesome.
It resonates with good vibes and general feelings of love, and due to this, is conducive to good times.