In my experience, 'moodiness' goes hand in hand with being wishy-washy, unreliable, unstable, and inconsistent in their feelings. I bring two examples:

Ex #1: social butterfly, swinging between manic activity/happiness, depression, and anger. (He was pretty extreme as he turned out to have bipolar, but regardless of that...)

Good mood: he was affectionate, 'loved' me, cheerful, full of initiative, etc.

Bad mood: he turned stone cold and indifferent, he lazied around, he hated everything and everyone, me included. He treated me like he couldn't stand me and got irritated by everything, annoyed by any of my 'needs' (e.g. simply speaking to him!). His affection evaporated.

Hand in hand with that, when he was Bored, it was a sign that the relationship was going bad and he didn't like me. It was like it was my fault because I wasn't 'entertaining' enough and didn't magically make his mood better. He would jump on any new girl around to chat because the novelty entertained him.

I call this the 'shiny new toy syndrome'. I was the old boring toy he had no emotional attachment to, and he latched onto any shiny new toy that could bring him a bit of entertainment.

Anger: lastly, he turned to having rage outbursts too, where of course I was the main target. When I stood up for myself against his neglectful behavior, he exploded into screaming rage because I wanted to 'control' him and 'take away his freedom'.

That was when he started turning very abusive and spiralling out of control, so it's slightly beside the point as a) it's an extreme case and b) we should walk away at the first 'rage' anyway.

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Ex #2: a very different person - reflexive, sensitive, calm and slow, a bit of a hermit. But in hindsight, I notice similarities...

Good mood: he was sweet and affectionate, bringing me little gifts/flowers, cuddling, etc. The relationship was going well; I was a good partner. I honestly believed he had genuine feelings for me.

Bad mood: he shut down completely, spent days alone, barely spoke to me. I thought he was dealing with his own issues but his affection for me was constant: I was wrong. It turns out he felt that the relationship was going badly, that there was something wrong with me. He turned cold, unfeeling, indifferent. His affection evaporated.

Hand in hand with this, he was Bored and it seems like he also faulted me for this. It was a sign that the relationship had run its course. I felt the pressure to be 'entertaining' to keep him happy.

Enter the shiny new toy syndrome: if a new person/girl showed up that was interesting and entertaining, he suddenly would be spending all his time with them, happy and egaged again, while neglecting me, the old boring toy. (more moderately than Ex #1; but the dynamic was the same).

Anger: While he never had angry outbursts, he eventually told me that when I told him about a behavior of his I found hurtful, his reaction was one of anger as if I was 'controlling' and 'threatening his freedom'. He never acted on it and worked through it aware that it was a trigger of his, and not actually my fault, but the instinctive reaction was the same nonetheless.

Conclusion:

Let's set aside the differences, which were quite dramatic, and focus on the similarities.

In both cases, their affection for me and their perception of whether the relationship was good or bad went up and down like a yo-yo following their mood swings. They were not constant and independent from their mood like in 'normal' people.

Their dramatic mood swings completely altered their behavior, their outlook on life, their personality, their relationships, and their feelings.

Their feelings were fleeting and superficial, mostly about whether they felt happy and entertained in that exact moment. If they weren't, rather than deepening the existing connection, they latched on any 'shiny new toy' that offered a bit of novelty and entertainment.

They were able to offer affection and thoughtfulness when in a good mood, but it became a chore they resented and couldn't be bothered with when their mood changed. I wonder if it was an 'act' of how they felt they were 'supposed' to behave with a girlfriend, rather than genuine. They became neglectful and indifferent. If I needed moral support or was sick, they were indifferent.

They were extremely self-centered and self-absorbed by their internal turmoil and unable/unwilling to consider anyone else's emotional needs.

They were selfish: happy to enjoy relationship perks - me going out to buy food, give a massage, etc - but never willing to return them (except occasionally when in a good mood).

They were, ultimately, unable to form a deep, meaningful connection, unable to go beyond a superficial level of 'feeling good in the moment'.

They were both similarly distant from their family, didn't have long-term meaningful friendships, didn't get attached to any place they lived in (hard for me to spot this red flag as I'm also a traveller; but I do have attachments and emotions to people and places, even though our behavior looks similar).

My mistake: was believing that despite the mood swings, their underlying core of emotions was stable and consistent. It was not. And that they were capable of a deep, permanent connection: they were not. It all came and went fleetingly with their mood swings.