Since Myrtlewing has trouble feeling emotion and love, how did he come to love Alderstar and his family?
oh I like this question!
Myrtle and Alder's relationship started with Alder realizing that Myrtle shared similar thoughts, and he became sort of dependant on the medicine cat (complaining to Myrtle in order to help deal with his anger at his dad and other Clanmates, for example).
Now, Myrtle (through not fault or control of his own) has great trouble feeling an affection towards others, and can pretty much only do so if it that itself affects him (not intentionally, it's just how it is).
So Alder liking Myrtle and depending on him = Myrtle has importance in this cat's life. He has a big part in it (this being a grandiose sort of importance, not self-doubt that would have previously made him think he wasn't important. He likes that he is, and that someone else feels the same way). And as Alder's and his relationship grew, so did his feelings to the point where he, too, depended on Alder.
Myrtle's support and love became like oxygen for Alder, and therefore became part of him. Alder's dependence and seeing Myrtle as someone great (even if he doesn't say it) in turn became part of Myrtle. So much so, that it becomes what he knows as real love.
He doesn't just love Alder because of how he affects him anymore. It's how it started, but it grew so much that it is real, two-way love.
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Hootpetal originally came as a connection to Alder. She's important to someone who's important to him, so she becomes part of his life. Then he became used to her until eventually he liked her as part of his routine, liked having her around and having her safe (plus he likes her humour).
And, also of great importance, she likes spending time with him. They enjoy talking with each other, and she also defends him when Alder (or someone else) can get a little too mean (even if Myrtle can't be bothered himself). It's the fact that she wants to talk to and defend him.
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Grousemane was similar to Alder (though less intensely) because he instantly made his crush toward Myrtle known and would continuously flirt and compliment him. Of course, Myrtlewing would like this because it made him feel great, and, like with Alder and Hoot, it would grow to the point of requited love and affection.
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With all of the kits, it was basically the same thing. They either first became connected to someone in his poly and through them became part of Myrtle, or depended on him (ex. Tree and Drizzle running to him when they have nightmares).
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I want to repeat that Myrtle DOES love them all, and not just for selfish reasons. That is how it started, because it's just. How he is.
It also helps that they CHOSE to make themselves part of his life. That was one difference (among many) between Alder and Nightfly, a vicitm. Nightfly's feelings were only made known to Myrtle after he already decided on killing him, and they were made to grow by Myrtle for his own gain.
Alder, on the other hand, chose to put himself into Myrtle's life over and over again.