Connotations of the word REACTIVE

Simply put, it's a neutral word. It can be either positive or negative as to effects or consequences. It is quick response (not necessarily premeditated or perhaps thought-out) to a stimulus of an event or circumstance.

See the following link for definition:
http://www.learnersdictionary.com/definition/reactive

In many contexts, reactive can be opposed by proactive. For example, business or lifestyle gurus often use these words to describe opposite ends of a spectrum of personality types or to describe types of business strategy.

Reactive strategies are those that respond to (react to) some unanticipated event only after it occurs, while proactive strategies are designed to anticipate possible challenges. Because no one can anticipate every possibility, no person or organization can be proactive in every situation. However, people or businesses that emphasize proactive strategy are usually more effective at dealing with challenges. (Allegedly).

If you don't buy this snake oil, many "proactive" people just look like assholes.

The negative connotation is not to the word "reactive" but to the word "reactionary", which means re-instituting the situation of the past, frequently by force, and being opposed to progress.

Reactive makes me think of something that contributes to a chemical reaction.