If it's someone that is naturally gassey, they get to a point where they no longer feel the need to hide it in order to impress someone. Like kozmicbluej said, it's the same reason that many females get to the point where they don't really care what they're wearing. At the beginning of a relationship, some people are obsessive about making the best possible impression on the other person. Females may never let their significant other see them without their makeup, hair fixed, etc. except first thing in the morning, and then they dash out of bed to brush their teeth and fix everything back up. Males may make sure that they're wearing clean clothes every single day, you never hear them burp or fart, and they may also be self-conscious about morning breath or about revealing certain body parts when they're not "ready for action", so to speak. Obviously these are stereotypical, but you get the idea. Over time, both people realize that their other half is just as human as they are, and stop obsessing over showing signs of being a normal human being...unfortunately, for some people this also means that basic manners tend to go by the wayside too.