Answer to Question 1
innate
instinctive
Answer to Question 2
Possible Response Points:
Resisting temptation is a huge part of making a long-term relationship work. Although people do not need to live cloistered lives, they do need to be aware that looking at other people as possible partners (even just imagining it) erodes the present relationship they are in. Guarding against such thought patters would be a helpful strategy.
Good relationships do not generally get better over time, but maintain their level of positivity. In contrast, once a bad spiral begins in a relationship, it is very hard to stop. So one important way to maintain a relationship is to be alert and ready to quash a negative spiral from starting.
Lasting relationships are often characterized by high mutual satisfaction, low available alternatives, and high levels of investment. If you want a relationship to last, you should behave in ways that appear rewarding and satisfying to your partner (and they should return the favor); you should avoid entertaining other alternative partners as possibilities; and you should invest in the relationship you have with your time and other resources. The more you invest in these ways, the longer and more satisfying the relationship should be.
Finally, it is helpful to cultivate an attributional style that interprets one's partner's good qualities as due to stable, inner factors and one's partner's foibles to unstable, external factors. This style is called relationship enhancing and is associated with longevity of happy relationships.