Karkat's in heat an Sollux can't resist the intoxicating smell he gives off.
Karkat and Sollux had been friends for as long as they could remember, since two sweeps according to Sollux. Karkat even helped Sollux when he was getting use to his psiionics. And now, six sweeps later, they had become closer...friendlier.... Karkat eventually worked up the courage to ask Sollux if he'd be his matesprit. Sollux, of course, agreed and they stayed together in Sollux's hive, living a more platonic life style than a romantic one.
That was until Karkat's eyes started changing from gray to red and he began going into heat.