The reason that love and affection are so threatening to someone with a dismissing attachment style is that these things were typically not made available from parents in childhood — even though on being interviewed, they usually state that their childhoods were idyllic, and that their parents were loving, without offering supporting memories of evidence. In this situation, the child will deny the need for love and affection rather than stay in a state of sadness and yearning. After years of pushing this lack of love out of awareness, the dismissing adult feels strong and confident.
But then someone comes along who really cares and says, “I love you.” And now all of that suppressed yearning wants to rush back from the suppressed past. But our dismissing friend cannot tolerate being so vulnerable and needy, so he feels angry at that reaction which threatens his hard-fought security, and he needs to push it away. So he pushes away the one who offers him love.