Ideally, we should be proactive whenever possible. Realistically, there are times when we all have to react to other situations. There simply is no way to anticipate everything you will come across.
The key to being reactive in a proactive way is to react to the situation and then to be proactive and take charge of the situation.
As an example, the tennis player might be able to return a hard serve but if all he is able to do is return with a soft lob, it puts the opponent in a position to win with the next shot. In volleyball, returning a spike with a soft return to the opponents setter will result, most of the time, with another spike, this time resulting in a kill.
In both of the situations described above, while the defending player(s) are reacting to the serve or spike, they are anticipating what the opponent will do so that while they are reacting to the opponent, they are also taking the initiative (by positioning their bodies in a certain way to force the opponent to become somewhat reactive themselves). By doing this, they are still having to react to the opponent but also are taking the initiative to try to be more proactive.
In businesses, there are times when everyone has to react (to competitors, to the economy etc) but those who react in a proactive way so they can take the initiative are the ones who will be successful in the long run
Have a great day!