I had the same feeling. It really changed the way I feel about time, and how I feel about losing those around me. It doesn't help that my mom passed literally a month before I played it, and I had just got over the anxiety of feeling like she was still here while knowing she's never coming back. Turning 30 next year also has made me focus on the past a bit more, and how teenage years have passed and how bittersweet it would be to go back. I would be able to relive the times I miss, or correct mistakes I regret; live out life knowing more of my potential. Although, it wouldn't be worth reliving all of the misery, loss, and boredom. It would ruin the meaning of so many relationships to the point that you would seek experiences elsewhere.
For Amicus it was grieving for a person I care about, grieving that he doesn't exist, and grieving that he mattered to me so much; that a person like him may not exist for me in real life. It also made me feel lonely for the first time in, well for as long as I can remember. I have always been okay with being alone, even fine with the possibility that I won't have a lovelife, but the few weeks after finishing Adastra, I just kept thinking about Amicus and pining for that representation for my potential love. I got lavender scented detergent specifically to get closer to that idea of "him." I guess it's a bit revealing that I do desire it regardless of how okay I am with being alone with the fact that I play gay visual novels, but I think most people here are in the same boat. We kind of have that loneliness or desire for someone that we can be attracted to, and vise versa.
There's the aspect for gay people that we get to finally be together, authentically expressing how we feel, uninhibited by external pressure to please others. How suffocating it is to constantly put your love on hold knowing that time is creeping up on you, getting your soon-to-be dashed hopes up, and having to obstinate from something you feel might pass you up. Not to mention we have less of a pool of people we can even be compatible with makes the odds of finding someone to love, especially with the climate and culture of gay people to have transient or non-committed relationships, even more bleak when you think about it. Dating apps, social media, or just chancing upon your dream man in real life just seems like a complete crap shoot. Adastra offers a glimpse at a seemingly unobtainable lifestyle, and it's still sad and full of heartbreak even when you get it.
Coupled everything with the fact that Amicus has a wolf design, and that canines have the representation for being man's best friend is also impactful. Dogs rarely live past 15 years, and it's rarely ever on someone's mind when you first get one that you'll likely outlive them 3 or 4 times over. A huge chunk of your life, sure, but still not enough. It jades the way you feel about getting another pet for awhile after they pass. It's a reversal of the roles we play in Adastra. We will be outlived, and we have to face the consequence of loving someone who may have to say goodbye to us prematurely. It's a eulogy at the beginning of a relationship, at the start of your happiness, and during the emotional height of your investment into the person you love. It's just sad.
If you do see this reply, know that someone did read it through and understand what you're feeling. I don't think about Amicus like I did, the feeling has passed like everything does. It just stands as a time that I was affected like many others like yourself by a story about a goofy, honest, and loyal wolf who we'll have to say goodbye to.