If you mind your Ps and Qs, other players will follow suit.
I always make it a point to say "hello" in a Duty Finder party, chat folks up, let them know if it's my first time in the dungeon or someone else's - that first level of communication coming from the healer tends to be really reassuring to everyone. I took a cue from FFXI and always say "thanks for the party!" before I leave.
One of my favorite videos in the last few years is Traffic Waves
- the idea that traffic jams are caused by people not being nice to each other, and a single nice person can unblock an entire snarled and angry mass of traffic simply by being polite and letting folks in.
I take that attitude in life in general, now. A single person not being a jerk can brighten everyone's day a little bit.