The problem was not that Edward never cried.
Winry had watched him cry after his mother's death until he had no tears left and the sobs came dry and hard and shook his small frame like the ravages of a disease. She'd seen him weeping brokenly after they'd survived her kidnapping, his hands over his eyes as if he could blot out a world where madmen stole children and sliced them apart in front of each other, feeding on their fear. It was her hand that wiped away the tears that squeezed grudgingly from his tightly-shut eyes, while the calibrating jolts rocked his nerves until they burned raw and he convulsed helplessly on the surgical table. And she heard clearly the tears choking at the back of his throat as he screamed his fury at the well-meaning friends who had kept the secret of Hughes' death from him.
No, the problem was not that Edward didn't cry. But his tears were always in and of the moment, a violent outpouring that occurred only when his emotions had swelled to the overflowing point. Edward cried openly and freely, but if given the chance to think first, he swallowed his tears, forcing them back down to distill inside him like the poisonous stone he was pouring his life into finding.
And that was what frightened Winry. Not the streaked and swollen face that followed his mother's death, but the exhausted and grimly calm one that he presented at her funeral, clutching his brother's hand and staring at nothing. Edward bottled up his grief until it forced its way free in those rare moments of mindless shock, bursting out in an explosion of pent-up stress and anger and pain, so that he cried until he could hardly breathe and then wiped his face and tucked down the corners of his dignity and started the whole destructive process over again.
She hated it, and that was why she cried for him, hoping that something in her tears would release the pain perpetually building inside him. He did it to be strong, she knew; to put on a brave face for Alphonse, and to keep himself safe from a world that preyed on the small and the weak. He was already small, bless his poor heart. He could not afford to be weak, and she knew it. But she could at least show him her own weakness; and perhaps someday he would understand.
With her, he was safe.
And with her, it was okay to cry.