step 1. he had fallen for her, but he is scared. she can't know. so he channels his love to someone else.
step 2. she falls for him. she gets jealous. he has someone else. he can't know. she channels her love to someone else.
step 3. he gets jealous, but he can't show. because she has someone else. he doubles his effort to someone else.
step 4. she is scared. she can't show. because he has someone else, and she has someone else. so she builds on her relationship with someone else.
step 5. he can't have her, but he's falling even more for her, but she can't know. they both have someone else.
step 6. she falls even more for him. but she what she knows is that he loves someone else. so she keeps herself away from him, and goes with someone else.
and it just goes on....
*security dilemma: a concept in realism
. "in the course of providing for one's own security, the state in question will automatically be fueling the insecurity of other states." "Security dilemma exists when the military preparations of one state create an unresolvable uncertainty in the mind of another as to whether those preparations are for defensive purposes only (to enhance its security in an uncertain world)....."(Realism by Timothy Dunne) so other states raise up their defenses, and in the process, makes the initial state insecure enough to re-raise it's defenses.
Major cause of it all: UNCERTAINTY