Great Sacrifices Will Be Inevitable
There is no such thing as a risk-free mission. However, given that no other human being has ever been successful in setting foot on Mars, the initial effort there will be an exceptionally risky one. Everyone who wants to be a part of the team needs to give it their all without any distractions, and that includes romantic relationships.
Simply put, romantic relationships are a distraction that can have a bad impact not only on the job at hand but also on the loved one who must remain at home. If Alyssa wants to maintain her focus on the ultimate goal, she will need to put off any romantic endeavors for the next 15 years and avoid getting involved in any relationships.