I enjoy playing tennis simply because it can be played individually against a single opponent 
I also like to play tennis, but I am not very good at it. The low cost of playing is a big draw for me.
Yes and it is pretty low cost, just a  racket that is strung with cord to strike a hollow rubber ball covered with felt over or around a net and into the opponent's court
We have parks here with tennis courts that can be used for free. It is a great workout as well. There is quite a lot of running and jumping involved.
Of course yes, you know you have to  play the ball in such a way that the opponent is not able to play a valid return.
There is a lot of strategy about placement. I like to do drop hits when the opponent is far back on the court. It forces them to sprint forward.
Of course yes. A lot of strategy is involved that is why it is played in the Olympic sport and is played at all levels of society and at all ages.
I sometimes watch the professionals. I am amazed at how good players like Nadal are. He is unbeatable on clay!