Because HE IS A JERK HAHHAHA
simply because of what he did to chase before, the couple wants to take revenge of him
You should read kiss me if you can...
Because of exactly what happened in kmiyc. (From what I remember Chase has no knowledge of this kidnapping plan.) That incident from the past left some severe trauma and deep wounds in Chase. And Josh is really pissed at Grayson (pissed should be an understatement, Josh hate his existence). So obviously the protective Josh wants to get back at Grayson even a bit.
Spoiler
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I don't really remember well but he is the reason why Chase hates dog and has a trauma about it and it was because Grayson made Chase think that he re*ped a dog while he was in his rut (which is not true) and because all his childhood is full of bad memories with Grayson like when he throw him from a chimney (or a high place
Can novel reader give me spoiler, please. What makes Josh and Chase want to kidnap Grayson? I know he is a jerk in KMIYC, but what did he do to make the couple decide to kidnap him?