Tidus has some notable dreams in the game that give us a glimpse of his childhood and his relationship with his father. Every dream or flashback has Jecht in it physically, or in conversation. I think Tidus dreamed about his father a lot because Jecht was Sin, and Tidus felt his presence when Sin was close. I think he clearly had issues with his father, and it's those issues that materialized in dream form.
In one dream, Tidus and Yuna are waiting on a boat. Yuna asks Tidus to take her to Zanarkand, and Rikku runs up exclaiming, "Hey! You said you'd go with me!" Tidus seems torn between the two when his father interrupts and says, "Stop dreaming! You with a woman? You can't even catch a ball!" Yuna and Rikku, in support of Tidus, encourage him to yell out that he hates his father. This helps him come to terms with his feelings.
That was Tidus' only dream that was completely in his head. His other dreams were flashbacks of real past events. Given that his mind formed the dream, one has to wonder, what does the dream mean?
I don't think that Tidus was debating between a romantic relationship with Yuna and Rikku. He hardly knew either, and he didn't look at Rikku in awe the way he did with Yuna. I really think Tidus viewed Rikku as a friend, or a sister. Yuna is clearly who he had the hots for. That begs the question, why were they both in his dream? Why was he torn about which one to take to Zanarkand?
I think more than anything, Tidus just wanted to get back to Zanarkand. Though skeptical, I think Rikku and Yuna both believed Tidus about Zanarkand, while Wakka and others felt sorry for him, thinking his mind was clearly playing tricks on him. Rikku and Yuna both expressed interest in Tidus' Zanarkand.
I also think Tidus felt lonely. He was away from home, lost in a world he didn't know or understand, and was desperately looking for someone to accept him, to believe him, to want him around. Wakka accepted Tidus, but didn't believe his story about Zanarkand. Rikku and Yuna did.
The third major player in Tidus' dream was Jecht, his father. I think Jecht being there is important because Tidus blamed his father for what was happening to him. At the time, he didn't know his old man was Sin, but he felt his father when he came in contact with Sin, and his father had disappeared, abandoning him. Auron had also given Tidus a sword from his father when everything went down.
Tidus' issues with his father would play a huge part in the journey Tidus was about to take, and although he couldn't know that, he felt his father. I think that, and Tidus' unresolved issues with Jecht had Tidus dreaming about Jecht again and again.