if ı remmeber correctly you need to find a enemy encounter.
sometimes enemys attack you with erotic attacks.
To better answer your question, there are far more methods you can go through to arouse yourself. Methods include drinks (lust tonic and devils milk), certain clothing options (vibrating panties), the method mentioned by Vhunter, certain status effects, ect. I have found that lust tonic is my most used method.