I agree for the most part with Loopy. But...I think that Dora had come to realise she and Steve could never be a couple no matter how much she loved him, and while she was jealous of Steve and Tina, there was no point making a scene, so she became wthdrawn and sad. It's hard to let go but she was trying very hard to; being grownup about it. When Joe came along - well-known, wealthy, someone of a background not dissimilar to her own - Dora's interest was awakened. It could never work with Steve, but here was a man who loved horses who was taking an interest in her...
I think Dora's happiness with Joe was like a knife in the heart for Steve. Despite knowing he and Dora could never make a go of it, he loved her. He mightn't be able to have her but the sight of someone else making her happy was too much to bear - even with pretty Tina to console him. That nasty laugh-!
Tina truly was the bridge builder. There was quite a deep chasm between Steve and Dora by the time Joe mentioned his girlfriend, and Tina could see they had very deep feelings for each other. I think she could have fallen for Steve, but she could see his heart was already full of Dora, and that Dora loved Steve, Joe or not Joe.