For researchers who work on second language learner variables like Elaine Horowitz:

Motivation is a consistently strong predictor of successful language learning.

(Horowitz)

Some learners have instrumental motivation: they need to learn for professional or other utilitarian reasons.  Others have integrative motivation: they develop an affinity for the target language culture which stimulates their language learning.

Activities

1. Read more: Motivation research
2. Play: Language learners' motivation