At first all Renji sees is outrageous hair, an outrageous sword, and even more outrageous power. They were hard to miss, after all. It takes a while to notice, but he quickly learns that there is so much more than meets the eye.
It was, in fact, the teen's eyes that he really took note of first. Aesthetically, they were rather typical, boring, brown eyes. But with Kurosaki Ichigo the phrase "the eyes are the windows to the soul" was so very true. Typically, Ichigo was all steely gaze and resolve, but it wasn't difficult to see any of the orange haired boy's other emotions either. Kurosaki was an open book and that's what Abarai Renji came to love most about him. Renji was simple in all things, so Ichigo's guilelessness was like a breath of fresh air in the Soul Society where everyone had an agenda. No matter what the situation, the young human was always honest and straightforward. The teen didn't know how to be duplicitous.
Renji adored that he could so easily read Ichigo. Anyone who took the time could do so. Granted, almost no one took the time to notice. Rukia called him fool and idiot when he was pensive. Kuchiki-taichou called him brash when he was determined and egotistical when he was confident. The captains that paid any mind to begin with saw only his youth, rather than his care worm and impossibly ancient soul. Most people saw austerity rather than the burdens and grief he bore. People saw the gruff fighter, but not the silent protector. Very few saw what a multi-faceted jewel the shinigami daiko was, taking only a surface glance at the young man.
On some level it made the sixth division fuku-taichou angry that so many missed how special Ichigo was, despite that several people, the people who mattered, did. It annoyed him that so many overlooked how much depth Ichigo had. It angered him that so many saw only a brash, cocky, idiot with too much power and more fight in him than was advisable. At the same time he was frightfully pleased that he had the real Ichigo almost all to himself.
Ichigo gladly gave everything of himself to everyone he deemed worthy and needed care. And everyone took what he gave, sometimes carelessly, sometimes gratefully. So many gave back with their power, but never with emotional support. They stood behind him because he could and would protect, but they would never stand beside him both becasue of the immensity of his power and because they didn't fully understand him. They didn't fully understand the heart of the reason for his power or his reasons to never avoid the fray, even when it wasn't his responsibility.
Abarai may never be able to match Kurosaki in power, but he could match him in emotions and reasons. After all, all he had was his emotions and his ambitions. He couldn't reach the sun the way Ichigo likely would, but he could reach for the ryoka himself. He'd never stop reaching for the ryoka boy's coattails. He been reaching unwittingly reaching for the boy since they met and couldn't bring himself to stop. Renji had once been the stray that could only howl at the moon; however, Ichigo had given him the courage to actually reach for that heavenly object. Ichigo seemed to do that for all those he encountered, which was the second reason he loved the young human man.
There were so many reasons that Abarai Renji loved Kurosaki Ichigo that he couldn't count them all. That was fine with him. They had eternity to learn about each other and count the ways they loved.