Let me try to take on this...
First encounter:
I think Cera was really trying to assault Littlefoot there after he had a good laugh at her after being sprayed in the face by a bug. It shows quite clearly, that she is brainwashed by her father to think high of threehorns but low of other kinds. Her pride was damaged there so she had to restore it by kicking Littlefoot's butt.
Second encounter:
I think Cera has too much fun playing in the bubbles to bother chasing Littlefoot off. Maybe she wanted to show off too there.
Sharptooth encounter:
I think this event is very important in the relationship between Cera and Littlefoot. Even though Cera doesn't show this, she must be glad about his mother saving her from certain death. Her pride just gets in the way.
They managed to escape together... I'm sure that such an event seals people and dinosaurs alike together a little

Encounter at the big underground:
As I mentioned, Cera probably knows that she's in Littlefoot's debt but - and maybe just because of that, acts as cold and mean as ever, also trying to show off which doesn't work

Encounter with Ducky & Petrie:
I do believe it is a coincidence that they run into each other to begin with. I do also believe that Cera has realised in the meantime, that finding her sisters by finding a way to cross the big underground is more than unlikely so she tried walking until she finds a way around it. As she fled from Sharptooth, knowing that he was on her trails, she meets a familiar face so she decides to push her reservations aside to come along. I'm sure she wouldn't admit it, but she was afraid of being alone, knowing that Sharptooth was around.
Finding Spike (deleted scene):
Cera has the most realistic and pragmatic opinion there. I cannot blame her for that to be honest. After all, Spike, after all these years, still often causes trouble (like eating vines they're trying to cross

)
Sleeping scene:
In the meantime, Cera has probably arranged herself with the group. She doesn't like them but sticks around because she's egoistic, only seeking the protection of numbers, but also, she suddenly has some people to look at her bragging and showing off.
I think she only tolerates the others sleeping with her because of sharing warmth. That, again, is only egoistic of hers.
Fight scene:
Well, Cera finally had enough of Littlefoot's leadership which didn't lead them anywhere. She is not to be blamed for that but for the things she, unnecessarily, says about Littlefoot's mother who once saved her life. She has reached a point where she doesn't see any use in the others anymore so she goes her own way, not bothering about the others (which is clearly visible when she doesn't check back on Ducky, Petrie and Spike)
Tar monster scene:
Cera realises that she's made a huge mistake and that she is dependent on their help after all. Maybe she even regrets putting the others into such dangers. I think the crying was mainly about the humiliation she receives

Sharptooth fight:
Maybe to redeem herself, maybe to show off or maybe just to get rid of Sharptooth (or a combination of those), Cera helps the others. However, when Petrie seems to be dead, she actually looks really sad which means that she must have cared at this point. That suggests that Cera must've realised how important those guys were for her and how much they meant for her while she was alone.
When they reach the Great Valley, they seem lile they're friends, just wanting to play together and everything seems to be fine. What does that suggest again? Well, there must have been a scene where Cera apologises (off-screen, I daresay it isn't a cut scene) and finally accepts the others as friends of hers.
Well, that is my take on this matter. It's just my interpretation which I've tried to work into one of my fanfics too (which I need to start uploading ugh note to self...

) Feel free to argue with me
