Something I found interesting reading Majjhima Nikaya 101, there is a little section where Buddha teaches against oneitis:

"Suppose that a man is in love with a woman, his mind ensnared with fierce desire, fierce passion. He sees her standing with another man, chatting, joking, & laughing. What do you think, monks: As he sees her standing with another man, chatting, joking, & laughing, would sorrow, lamentation, pain, distress, & despair arise in him?"

"Yes, lord. Why is that? Because he is in love with her, his mind ensnared with fierce desire, fierce passion..."

"Now suppose the thought were to occur to him, 'I am in love with this woman, my mind ensnared with fierce desire, fierce passion. When I see her standing with another man, chatting, joking, & laughing, then sorrow, lamentation, pain, distress, & despair arise within me. Why don't I abandon my desire & passion for that woman?' So he abandons his desire & passion for that woman, and afterwards sees her standing with another man, chatting, joking, & laughing. What do you think, monks: As he sees her standing with another man, chatting, joking, & laughing, would sorrow, lamentation, pain, distress, & despair arise in him?"

"No, lord. Why is that? He is dispassionate toward that woman..."

The ...'s are the translator's, btw, not mine.