I wasn't aware of the strength of that phrase until I used it in a conversation with my wife. It led into a much more involved discussion than for which I had bargained.

Into the minefield
It turns out that the phrase was a stock rejoinder her former husband would use as a defense when he was caught out about something. When I used that phrase (for roughly the same reason as her ex) it brought back a flood of negative emotion in her. I was then the happy recipient of that negativity.

She explained that to tell someone to 'don't feel that way' is to negate their emotions, their freedom. My initial reaction was she had probably read one too many articles in Vogue where they were dealing out feminist claptrap. Upon further reflection however, I came to the conclusion she was absolutely right.

Reasons why
Emotion is a response to a stimulus. When someone does something you find pleasing it engenders the feeling of happiness or satisfaction. When someone does something that elicits a response of anger or unhappiness it too is a chemical reaction to a stimulus. To tell someone to feel one way or another is to deny their humanity,their legacy as emotional creatures. The negative reaction from the recipient of a stimulus can usually be anticipated. In simple terms when you do something stupid you're going to get a negative response. Telling someone to either feel a certain way or not to feel another is an attempt to shift the responsibility from you (the stimulus provider) onto the recipient of the stimulus.

Now we're making headway
This made sense to me. I try to be intellectually honest with both myself and others. I realized that to flippantly say 'don't feel that way' is a copout. That is assuming of course that both parties to the interchange are reasonable and relatively sane. I am very careful to not use that phrase anymore not only because it opens a dark well into her soul but because I finally realized it's a dishonest way to deal with another human being.

I suggest a more honest way to proceed is to ask why the other person feels the way they obviously do instead of discounting their reaction. I've learned to deal with the stimulus I provide instead of the result (over which I have no control). What I do have control over is the stimili I create. When I don't do foolish things I don't get negative reactions. When I take others feelings into account it provides positive feedback to me. When I do things that are unthoughtful or even selfish the reactions are predictable. Part of becoming a mature human being is accepting responsibility for what you do including the things you do that effect another persons emotional life.

Disclaimer: just because you think it's over doesn't mean it's over
Part of learning to live with another person is negotiating through the minefields left by former combatants. These emotional bombs can be set off to create havoc by a simple misstep, a chance wandering off the established path. The goal should be to either learn to not trigger landmines or to defuse them. I suggest the latter as simply avoiding them leaves them intact and ready to do damage. Defusing them by discussion and understanding removes them as a threat. The work involved in doing these things also leads into a much fuller understanding of the other person, and that's a good thing. Be aware however that just because you remove a mine doesn't mean there isn't still a bit of explosive in the hole. Not enough to maim, but certainly enough to get a nice burn.
It's a work in progress