The Romance of Tristan (Parts 7-13)

While Isolde, Tristan, and Governal are in the woods the narrator tells us that Isolde's mother only meant for the potion to last three years. Her mother might have been thinking of the common things back then, like providing heirs. It's probably much easier to conceive a child if the two partners are, or think for that matter, that they're in love. The women in Isolde's family are thinkers after all.

So when it wears off, all guilt associated with their relationship comes back, as if the potion had somehow blocked their guilt. And so they repent, Tristan allowing Isolde to return to Mark after writing him a letter with the help of the Hermit.

This represents the effects of true love, where there are second thoughts about what the couple is doing and why they are doing it. They question whether this is real or just a physical manifestation of lust and if it's in the right or wrong of their current society and moral standing.

But perhaps, the way their love was, said love potion was not a love potion but a lust potion. But that's countermanded by the fact that they felt no guilt. Lust is a sin, especially back then, and if they were to feel it, the thought would strike fear into the two lovers. Though whether the lack of guilt the two showed is a effect of the potion is never stated.

However, they still have a fondness of one another, and do still love each other. So maybe, the potion was a catalyst for a spark between the two. Something that was already there.