Usually an affection tourist will latch on to those who invest in the tourists well-being and/or emotional state. It is rare for an affection tourist to invest back.
Sometimes an affection tourist will target someone new when they need something, like they break up with their partner and need some sympathy. In this situation, they will be super friendly to the target, becoming their temporary best friend until their affection needs are met and move on to the next target, usually quite suddenly.
Once the continued affection is depleted/starts to wane, or someone else offers attention, the affection tourist moves on and barely speaks to the previous person for some considerable time, not out of dislike for that person but just because they can't be bothered as that person has nothing to offer the tourist.
"James broke up with his boyfriend last week. I hadn't heard from him in months, but suddenly he's all about hanging out with me and doing stuff. It's great but I'm not holding out much hope of it lasting because he's just an affection tourist. I'm sure he'll find someone else to be best friends with soon."