Typically there are two ways for trolls to be born, both males and females can have eggs either way.
The first way is the more popular choice, where a couple or single troll decide they want a child and then concentrate so much love and care into what most would describe as manifestation.
In a couple days you should be able to know if the mother or the father are growing an egg in their hair.
The second way is the less popular option but still viable. It always requires two trolls so it's typically a couple that has a baby this way and only male trolls are able to impregnate someone else so it's usually a female that has the baby.
Sure there are male and male couples who choose this way and sometimes female and female couples will find another male troll to bring into their relationship.
Using this method is not always guaranteed to be successful though so trolls still have intercourse with there partner for a merely pleasurable reason.
And there are ways to prevent it from happening this way as well.
While most trolls have the decency and heart to not force themselves upon other Trolls, rape does still happen once in a blue moon on the troll tree.