How we cite our quotes: (Chapter.Page)
Quote #1
By mistake, Brangain brought the love potion and handed it to Tristan, who drank and passed it to Yseut. Both thought it was good wine: neither knew that it held for them a lifetime of suffering and hardship and that it was to cause their destruction and their death. (1.44-45)
According to this quote, once Tristan and Yseut drink the love potion, their fate is sealed. There seems to be no opportunity for Tristan and Yseut to make a choice to love one another, or at least, to act any differently from the way they eventually do. The love potion symbolizes both powerlessness to resist love and powerlessness to change your fate.
Quote #2
Hear now of the hunch-backed dwarf, Frocin. He was outside looking at the heavens, and he could see Orion and Venus. He knew the courses of the stars and he observed the planets. He knew what was to happen in the future: when he heard that a child was born, he could predict all the events of its life [...] He looked at the conjunction of the stars and his face flushed and swelled with rage, for he learned that the king was menacing him and would not rest until he had killed him. (2.54)
Frocin's ability to predict the future from reading the stars seems to eliminate the possibility of free will, since the events of a child's life are determined from the moment he is born. Yet Frocin's response to the future he reads for himself—flight to Wales—suggests that if you know your own future, you can change it.
Quote #3
'Indeed, sir, you do not know the reason for her love for me. It is because of a love potion that she loves me. I cannot part from her nor she from me. That is the truth.' (7.79)
The removal of choice from Yseut's love for Tristan also means that Tristan does not do anything to deserve it. His understanding of Yseut's reasons (or lack thereof) for loving him is interesting in this context, in which a holy hermit has just asked Tristan to turn to God. That's because in traditional Christian theology, what makes God's love unique is that it, too, is totally unconditional.