I'm trying to create alien species that is hermaphroditic, with strong sexual conflict. In essence each being tries to trick or force (by mechanical or hormonal means) its partner to take role of female during copulation, as to avoid costs of bearing the young. Their behavior is in this case is similar to snails or flatworms.
The question is: what kind of environmental pressures favor hermaphroditism over regular sexual reproduction.
Can very unstable environment that requires fast breeding or makes meeting other members of the species be such pressure? Any others?
Update: after reading answers I think I am going to change my Herms.
They evolved from hermaphroditic ancestors. In fact hermaphroditism is now dominant on their homeworld by some evolutionary accident. That removes the problem with establishing how they became hermaphroditic in the first place.
Each Herm has both female and male gonads. During copulation they exchange complex cocktail of hormones trying to suppress sperm production and force ovulation in each other. The winner forces its partner to became pregnant and goes looking for another occasion. It creates evolutionary arms race. Winners produce lots and lots of children.
There was once isolation in time between tribes. Herms' homeworld has short good seasons. Winters and summers are harsh and most lifeforms spent them in cryptobiosis. Now, when climate was tamed this ancestral ability helps Herms in interplanetary travels.
Bidirectional hermaphroditism is the basis of social development. Some subtype of hormonal cocktail may be used to control subservient members of the tribe. Later in industrial age some chemical tyrants may order production and dispersal of such hormones on mass scale. They are called husbands of a nation in symbolic terms. Their tyranny lasts until some of their subjects acquire resistance. Herms' warfare is rather slow, sneaky and eugenic.
There are also other types of social arrangements. Rare parthenogenetic clans specialize in science, philosophy and religion. There is also quite new development - participatory relationships where partners use modern medicines to ensure equal exchange of genetic materiel.
Thanks everybody for help.
Update 2: I forgot to add. Herms are not humanoid. Their bodyplan is roughly similiar to them: https://en.wikipedia.org/wiki/Triops, more vertical, with longer legs. This may change.