For me, it's mostly because of my social circle. Since high school I've had many Vietnamese friends so I have always had the language being spoken around me a lot. Also, for some reason many Vietnamese people don't see it as rude to speak Vietnamese around someone who doesn't understand it even when there is a common language like English that we all speak and understand. So over the last 8 years or so I've picked up many common words and phrases from them. A few years ago I also spent 6 weeks in Vung Tau with a couple friends which was a great experience. Absolutely loved it. My plan is to visit the country every two years or so for vacation.
The thing is, I've had incredible difficulties learning the language. Despite being exposed to the language a lot and at times putting in real effort to learn it, I've had little success. The most progress I ever made was during my stay in Vung tau where I enjoyed complete immersion. I've identified two main reasons for my difficulties:
1. The tones. Predictable, I know. I've had to put in incredible effort just to be able to pronounce some very basic words and phrases half-way correctly. It gets really frustrating when people don't understand me speaking very simple sentences.
2. Almost all of the Vietnamese learning materials I have found have been for tourists and very badly organized. Nothing comprehensive. Although that Japanese guide for Vietnamese that someone posted in another thread looks promising.
I'm going to see how far I can get with any useful learning materials I can find. Every extra bit I pick up is rewarded many times over again by improving my understanding of what's being said around me.