Sometimes division seems good and constructive. For example, living in neighborhoods of the same denomination or creating schools for specific types of backgrounds may seem like it's for the good.

But already at the very start of the creation of the world, the Sages of the Land of Israel impart to us that division is extremely undesirable even if it is not the product of strife or disagreement.

Rabbi Yochanan (Midrash Rabba) teaches us that the reason that the phrase "It was good" is absent on the Third day of creation is because the separation (or division) of the waters into the Sky and Seas was mentioned. So averse are the Scriptures to the mere thought of division that they refuse to apply any sort of approbation when division is mentioned.

The lesson that the Sages of the Land of Israel teach us is that division, even if constructive and necessary is ultimately negative and undesirable. It should never be the first choice nor the long term solution.