Why when I only sleep respecting the Sunnah I do not dream about a girl that I loved so much, otherwise I am dreaming about her every night without exception and she always appears in my dreams on my left side, never on the right. Please could you help me explain this because these dreams are most of the time nightmares and cause me some depression in the morning after?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Since you said that when you sleep according to the Sunnah, you do not dream of the girl you love, then this is evidence that dreaming of this girl at other times is from Satan. You have also to know that a Muslim is not allowed to establish any affectionate relationship with a girl except within marriage regulations. As for nightmares, the Prophet said: “The true dream (vision) is from Allah and the bad dream is from Satan (devil). So, whoever sees a vision he hates, he has to puff out on his left three times. He has also to seek protection from Allah, then what he saw in this dream will not cause him harm and he shouldn't tell it to anybody. And if he sees a good dream, he should tell it only to those whom he loves.” [Abu Qatadah]

Therefore, there is no need to annoy yourself by these dreams, especially if you follow the Prophet's directions by puffing out on your left and by seeking the protection of Allah. Puffing out means blowing air gently without saliva. Moreover, we draw our brother's attention to reciting Ayat Al-Kursi [Quran 2:255] as it has been narrated by Abu Hurayrah in his long Hadeeth that if a person recites it when he goes to bed, he will be protected by Allah from Satan till the morning. This Hadeeth was narrated by al-Bukhari and others. We also remind Muslims that they should perform ablution and recite the proper Dhikr (remembrance of Allah). He should also remove all bad things from the house. Doing the above will please Allah and the pleasure of Allah results in goodness in this life and the Hereafter.

For more benefit on the rulings of love relations, please refer to Fatwa 84444.

For more benefit on proofs prohibiting mixing among the sexes, please refer to Fatwa 84281.

Allah knows best.