When someone lightens up your life just by their presence, you can be sure they are emitting a very positive energy. When someone puts a damper on the meeting or a conversation, you can be sure there is some hidden inner baggage which contains sorrow and darkness. Which do you bring to the party? A little light or a little dark? A lot of optimism or a little pessimism?
In olden days, before one switch could illuminate a whole city, one candle had to be lit by another. So if you are aware that you are carrying a little darkness, perhaps it may be useful to spend time with someone whose inner light is already lit, and stays lit in all circumstances. Eventually we may learn the art ourselves, and then serve as a candle to others.