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Welcome to the wonderful wide whimsical world of being a man who doesn’t just wear a bra but needs a bra lol. It can be an adjustment for sure at first but as you noted after a few days it’s clear going braless is difficult.
I’m a believer that as we wear a bra, it protects our breasts from the elements. Fabrics, weather, mobility, etc. having our breasts is like having our hands in our pockets. If we put our hands in gloves or pockets for awhile our hands feel softer, relaxed, warm etc and then when taking them out or off, and are immediately acknowledged to the temperature shift or elements as we use them in exposed fashion again. Same with breasts. As we protect them, the more blood flow is brought to them by being in the cups, more warmth provided to them by the bra, more protection from elements to areolas and nipples and more shaped by the bra. Which in turn make our breasts when we take the bra off. Appear more feminine in shape and contour due to being molded for time of wearing, areolas and nipples appear puffier, and so forth.
Just my theory.