After he finds her hidden hentai manga...

"Tama" enjoys reading naughty manga stories, but...
One of those manga (doujinshi) is found by her childhood friend "Suzu."
Thinking that he will be critical of her hobby, she is saddened...
However, surprisingly enough, he instead tells her that it's nothing to be ashamed of.
Which he immediately follow up with a confession that he enjoys naughty things too... and a kiss.
Unable to resist her curiosity towards sex, she spends the night with him...
and thus triggers the beginning of his obsessive love for her.