I like the way the cited site says this:
The word “reactive” implies that you don’t have the initiative. You let the events set the agenda. You’re tossed and turned, so to speak, by the tides of life. Each new wave catches you by surprise. Huffing and puffing, you scramble to react to it in order to just stay afloat.
In contrast, the image we associate with “proactivity” is one of grace under stress. To stay with the previous analogy, let’s say you’re in choppy waters. Now, you look more at ease. It’s not just that you anticipate the waves. You’re in tune with them. You’re not desperately trying to escape them; you’re dancing with them.
Proactivity is a requirement for being effective in life and also for being able to create a great and happy life. One cannot and will not succeed in life without it. Period!
Basically, proactivity and reactivity are created from different viewpoints.
"Pro activity" is activity that is initiated by the person involved, using anticipation of what could occur and deciding what to do to create the desired results. The most effective people in life, obviously, are proactive. (Stephen Covey, in his book The Seven Habits Of Highly Effective People, include it as Habit 1: Be Proactive - click on the link and read it.)
Reactivity is a non-activity, in a sense, where circumstances and/or other people do something and then one automatically, without thought, reacts in response to what occurred. It is not internally motivated and is primarily a response to what is outside oneself. It is the opposite of self-determination, as it is other-determined.
A reactive person is the "victim" of what happened and/or of other people. The proactive person is the "cause" of what is occurring in his/her life; he/she solves problems, rather than letting them recur over and over "as if a victim" of them.
Even people who are quite proactive in some areas, such as highly accomplished executives, are reactive in others. Many so-called powerful people get angry or let their health, relationships, or other areas go downhill - they are, in those cases, proving themselves to be "reactive", the victim of whatever occurs. A great discussion of this, which also applies to any individual whether in business or not, is in Tony Schwartz' book "The Way We're Working Isn't Working", where he deals with the four forgotten needs that energize great performances and great lives.
Which do you choose?
Stephen Covey, in his classic all-time bestselling book The Seven Habits Of Highly Effective People, is the primarily remembered source of the discussion of being proactive. Robert Rutherford first taught me this concept, in addition to the four quadrants of importance versus urgency
I also liked this one:
Whether you choose to believe it or not, being proactive is among one of the most important traits of successful people. It allows anyone, in under any circumstance, to choose what kind of life they have or will have in the future. This is a far stretch from reactive people, which in my opinion, are the exact opposite.
The difference between proactive people and reactive people is that proactive people take action, they take initiative, and they come up with ideas to get things done despite the conditions they are in. Reactive people, on the other hand, usually are the ones to react to a situation rather than take initiative to create a situation. Thus, they are constantly under the influence of the conditions around them, stopping them from taking action and getting things done.