Why does my ex cheat on his gf with me, he acts like hes completly in love?

me and my ex were together for a year and broke up with me for his current gf. 3 years ago. we stopped talking for a year then became friends...and we started making out...messing around...ect. he doesnt speak about his ex but im uming there very muvh in love...3 years and he picture is his wallpaper on his phone. i say u have a girl friend...he says why do u care...he treats me like im the only girl. and we hang out for 3 or 4 hours in my car jsut talking about everything. what is this> what am i> are they in love? are they like jim morrison and pam? i just dont understand, help me