Motivation can come and go - like the change in seasons or ebb and flow of the tide.  So may your reasons for participation in activity.  When you were a child - you enjoyed to "play" with others.  As a teenager the social significance may have been replaced with dreams of competition.  Adulthood has seen the concern for your health motivate you.  Well, why not all three - use what ever reason you can find to remain active.  Better yet, employ several reasons. 

So many strategies, many reasons to motivate or move yourself!