[SPOILERS S3] What is Jonas and Martha's motive?

Stupid question (first time watcher) but why don't Jonas and Martha, if they love each other, just be together? Why would Jonas as Adam want her dead and to end the knot? Who cares about the apocalypse and all that? Just BE together and what will be will be. And because Adam wants to destroy this knot, Eva is doing everything in her power to maintain it. I just don't get it. WHAT IS THEIR MOTIVE?