It hadn't always been this way.

Even during the war where friendships and blood ties were
 forged out of necessity and proximity, the pilots of Wing and 
Shinigami were never so close.

Heero, driven by the ideal of the perfect soldier and dreams of
 a past he couldn't remember, struggled to balance his training
 with the vague blossomings of newly awakened feelings within him.

He hadn't been much better off, possessed by a past he remember 
all too well and relived with a bitter sense of recognition in his free
 moments; memories of blood, smells of soot and burning, the sour 
taste of ashes and regrets lodged deep within his senses.

If there was an attraction, it was repressed.  Sudden moments of 
friendship were deliberately shunted by either side.  The idea of
 becoming close to another person, another soldier, both repulsed
 and fascinated.  And it was war time, too dangerous, too close, too
 *simple* was the litany that ran in both their minds, brought out, 
constantly repeated, and held close instead of the warmth of each
 other.  When a look, an unguarded smile, the truth of feelings spilling 
out of eyes grew too much, they clutched their empty words and warnings close.

By then, the war was more of an excuse than a reason for avoiding contact.

And when the war was over, there was nothing, or according to reason there 
*should* have been nothing keeping the two from reaching out.  Nothing other
 than  habit and fear.

Duo had never considered himself a coward until now.  He was Shinigami, ever 
laughing, bright Death who rejoiced in the demise of the soldiers and hidden 
daemons who *dared* to touch the bright lights in his life with their blighted touch
 crumbling all shining dreams and hopes to the grey of ashes.  He was re born from
 the aftermath of that violence after all.  What fears could Death have?