Jiu-Jitsu is a contact sport. Contact in this case is an understatement. We are constantly in contact with our training partners in some sort of way. We get in all kinds of positions that if they were taken out of context, would be very damaging to our sexual orientation or relationship with a significant other. This close contact creates all kinds of moments that you would normally avoid like the plague in real life. Sweat, body hair, smells, etc. , are part of what we do.
We also talk a lot about how being respectful of the danger you put someone in when you have them in a submission and how you have to trust that your training partners will have good judgment and will keep ego's/aggressivity in check by not cranking too fast and hard on those submissions.
That said there will always be THAT guy that is over aggressive or has really stinky gi's or that thinks deodorant is a small country in Europe. Don't be THAT guy! North South Jiu-Jitsu Underwear ambassador Tom DeBlass has great insight on hygiene, and Thomas Nadelhoffer wrote a great piece on overly egocentric training partners. We are all guilty of breaking some of those unwritten rules from time to time but we should always strive to be our best and earn respect by being as respectful as possible.
That attitude gives us the ability to learn much faster. Rolling with fear of not being able to work the next day from an ego induced injury or that we roll with the stinky guy, puts the breaks on our learning curve and our confidence to go for techniques that might get you in danger or in compromising positions. There's also a certain confidence it brings to us when we know that we have a perfectly clean odor-free gi, fresh underwear and no body odor. Going north south, pulling the guys head in a triangle, smashing in high mount become positions of choice.
So be trusted by respecting your training partners with a great attitude and good hygiene.