I find this whole "tracing special interests" topic pretty fascinating. Now I've had numerous fixations and obsessions in the past, so I've been able to sort of figure out what seems to be going on.
Now from what I can tell, the way it works is that I've got everything related to living and the daily routine (Stuff like should I study? I Should do my requirements now? How should I organize the bed I call a nest? How do I like my food? Should I talk now? and stuff like that) in a single irreplaceable and immovable hard disk (my brain is the computer). Then there are a number of ports to insert the topics to be fixated on, of which there are many. Often many ports have certain "themes," such as mecha and robots, which get switched out depending on what I find fascinating from one time to the next. For example: I first stuck with Gundams from Gundam 00, Macross with their ridiculous mecha came up and I wanted to fuse them (Gundam 00 was still there, it just took a backseat to Macross), then Unicorn came along and I wanted to fuse that too (Macross joins Gundam 00 in the backseat), then came Knights of Sidonia with their sciencyness and I wanted to add that too (Again, more in the backseat), and then finally a new port with 3 themes mixed together (Mecha and robots, ridiculous "biology", and power) grew and replaced those ports, the fixation within being "Becoming a Gauna-Human Chimera from Knights of Sidonia plus the composite abilities of all Gauna like the Black Gauna's ridiculous speed, power and reaction time with the general shape and form of Accelerator with his Vector Control and Deflection ability"(Gauna-Human Chimeras are Garde sized, so like 17 meters tall I think, and are kinda basically bio-mecha, even if that "biology" is complete hax).
How new fixations get added really depends on chance and the fixation before them; sometimes I just find something fascinating enough to look up everything about them, but most of the time it depends on even the slightest connection between the 2 topics; case in point: I was looking up the feats and powers of Asriel Dreemurr and thus his connection to the theme of "power," but was then brought to Accelerator when I saw that they were both basically albinos (I say basically, but it's a bit more complicated than that), that held immense power. Accelerator's kind of power (reality-warping psychic shenanigans), combined with a personality that resonated with me and a general feeling of ability and power even at his weakest won me over, and since then an amalgamate that's predominantly Accelerator become my official persona (The ports he fit together were: Psionics/Psychics, Power, Albinoism and the white hair and skin that came with it, because White, Black and Gray are my Tier 0, absolute favorite colors, Control, Insanity and Mental Instability, Androgeny, Willful Isolation, Growth, and Neutrality). Now the general idea is that the more ports and themes a very specific topic covers the more it'll stick around, with a lot of mixing and separating and mutating in between.
