(For some context, see the previous entry, in which this was originally a sidebar before it got too long.)

To put it one way, anime almost always gives you an external view of characters and events, not an internal one (one where the character's internal feelings and motivations are clear to the reader). Manga and games let you give the reader a somewhat more internal view, and written text lets you give them a fully internal view if you want to.

(You don't have to, of course; lots of written works are told from various detached third-person perspectives.)

In a work written with this internal view available, the external events may not make sense or may not be sufficiently convincing by themselves. For example, it might only be clear how and why two characters fall in love from an internal view; if you just have the external events, it seems implausible or stupid. Since anime mostly strips away this internal view, you are left only with the unconvincing external view; in order to make the story make sense again you may need to add or change external events to be more convincing and to better illustrate the internal story.

(The other version of this is when a character is more sympathetic or interesting because you see their internal perspective; if the story is told purely from an outside perspective, they wind up looking unpleasant or nasty or bad.)

There are at least two ways for an anime adaptation to fumble the necessary story changes: it can simply do a bad or unconvincing job of them, or it can do a good job of them that still doesn't fit in well with the other external events and feels out of place, like something awkwardly grafted in.

(The worst case is stories where the internal view is an intrinsic part of how the story works, for example where the story being in first person narration is fundamental and it would be very different in third person. But I suspect that those don't get anime adaptations very often or in fact at all.)