system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does it takes 7 months to reach Mars but 2 years to come back?

If humans are to travel to Mars using today's technology, it would take them 7 months to reach Mars and another 2 years to come back. Why does it takes longer to come back? Source: strip the cosmos. Saw it on discovery but they didn't really explained why it took longer.
gpt
The Earth and Mars are both orbiting the sun. Earth makes more trips around the sun then Mars does in a similar time period.  As a result if you launch at the perfect time to transfer to Mars, by the time the craft got to Mars, the Earth would have moved out of position for the perfect trip back. 
So you either have to wait for the planets to align again or you take a less than optimal path back.