You're of course framiler with the phrase "When it rains, it pours." I think this applies many times to after you find yourself with a girlfriend. The cause is likely because you're no longer looking for a companion so alot of the pressure many people have in terms of interacting with the opposite sex have been relived. You now have a companion so you're not really on the hunt anymore. You can go to the bar or club whatever with friends and you're not really looking for a girl to hang out with and it shows. You're relatively calm your smiling having a good time, loose, fun loving, no worries or cares to speak of. Girls can see this on you and are attracted to this because you're not trying too hard to make an impression.
When you talk to girls in this state, you're able to conversate well because you're not focused on getting in her pants, you're just making small talk with some person. Girls are comortable around people like this and this generates intrest and attraction. If one could attain and maintain this state without a girlfriend he'll obviously meet with the same success. This is why we tell people just to go out and have a good time when they're at clubs or bars or lounges instead of focusing so much on how to achieve through strange and sometimes silly tactics and moves while socializing.