u/Cheeseanonioncrisps

In the Land Before Time films, Spike is very clearly coded as having some kind of developmental or cognitive disability. He's basically nonverbal (although he is shown to be able to say single words, usually in moments of extreme emotional distress) and regularly seems to struggle with understanding and paying attention to things that the others find simple.

It's actually pretty good representation for kids with these kinds of disabilities (especially given that the first film came out in 1988). Spike being different is basically never treated as an obstacle to him going on adventures and, although the others sometimes get frustrated with him, he's still treated as a valued member of the group.

Obviously, it's entirely possible that Spike's condition could be something genetic that he was born with. However, I think there's a more likely cause: Spike is malnourished, and has been since he was hatched.

The thing with Spike is that he's a Spiketail and Spiketails eat a lot.

And I don't mean by this that stegosauruses, like the other large species of dinosaur we see in the show, would have eaten a lot of food. I mean Spiketails, the fictional version of stegosauruses from the Land Before Time-verse, canonically eat more than all other herbivores.

Like they somehow eat more than the Longnecks, who are several times bigger than them.

This is shown in one of the better later films, The Big Freeze, when we finally meet some other Spiketails and learn that the reason that there weren't any in the Great Valley up to now is because they're forced to be nomadic.

They cannot stay in one place like the other species we see because they need to be constantly moving from place to place to get more food.

The mother Spiketail who offers to adopt Spike is literally shown reminding her daughter to “bolt your food so you can eat more”.

Later on, when the weather turns and they leave the valley (explicitly because they're worried about exhausting the food supply if they stay) they get less than a day out without finding food and act like they're literally about to die from starvation. They end up following a nonverbal toddler into a literal blizzard because they think there's a chance that he might smell food out there. It's insane.

Spiketails in the Land Before Time verse are either horribly inefficient energy-wise, or have shitty digestive systems that mean they poop out most of their food whole, or both. Even in times of plenty, they literally cannot afford to savour food too much because all that chewing is a waste of vital calories.

Now consider the environment of the first film.

The whole point of the first film is that the dinosaurs are all trying to reach the Great Valley, not because it's predator free, but because it's the only place that they know will have food. There's been some kind of environmental disaster (possibly Chicxulub related??) that is killing off all the plants.

Spike's egg is found in amongst a patch of tall grass that he eats the moment he wakes up. All the other grass is gone. One can imagine some poor Spiketail family eating around their last egg, knowing that they'll have to move on when the food supply runs out but wanting to give their baby the best chance if it doesn't hatch in time.

All the dinosaurs are shown to go through a brief 'baby' period when they first hatch, before settling down what looks to be a few weeks/days into the mindset of young children.

Spike doesn't properly develop out of this stage, however, because he's gotta be constantly starving for those key developmental days that the first film covers. The other child dinosaurs, who aren't horribly inefficient eating machines, definitely express hunger at various points. Spike's brain would have been diverting all nutrients away from development and towards running away from Sharpteeth.

But what about when he gets to the Valley? Surely he'd have enough food there?

Well yes, but the thing is that Spike is being raised by Swimmers/Duckbills/Big Mouths (it took a surprisingly long time for them to settle on a name for Ducky's species).

There's enough food around for Spike to start making up for lost time (although he'd probably still have to deal with after effects from his early starvation) but everyone around him keeps expecting him to eat like they do.

The other kids regularly comment on his eating habits and keep dragging him off to play or go on dangerous adventures to places where there isn't as much food. Remember, baby Spiketails living as nature intended are literally taught to scarf down their food so that they can maximise their caloric intake. Spike, for all his reputation as 'the greedy one', probably has his species' equivalent of anorexia.

This also explains one of Spike's other notable traits: he's sluggish. The baby Spiketail we meet in Big Freeze is very energetic— as you'd expect from a little kid— and the adults in the herd move about as much as the adults of the other dinosaur species. Spike, on the other hand, is quite lethargic. He doesn't run around as much and seems to sleep more than the other dinosaur children.

But of course he does. He's exhausted. He's being taught to live up to the standards of a species that he doesn't belong to, and it's putting him in a situation where he has probably literally never had the right amount of calories in his system.

Spike's adoptive family mean well, but they are inadvertently hampering his development and likely dooming him to health problems later in life and a shortened lifespan overall.

Probably should have just gone off with that nice Spiketail family after all.