I wish you had not asked me this question. But since you have, here is my explanation (people here do not agree with it). Aryans lived in a region where the sun never set for seven months and faltered only in the eighth month. RigVeda says there are seven sons of God-Mother Aditi and the eighth, Martanda, was born undeveloped. The eighth was used by the Gods to create humans who will be born and die. So, seven became sacred. Seven heads of Airavata, the elephant mount of Indra; and seven heads of the horse, Uccaishrava, the mount of the Sun God; seven are the primordial sages born in each age; seven manifestations of pranas; seven types of officiating preists (Ritvijah); etc.
अष्टौ पुत्रासो अदितेर्ये जातास्तन्वस परि । देवानुपप्रैत सप्तभिः परा मार्ताण्डमास्यत ॥
सप्तभिः पुत्रैरदितिरुप परैत पूर्व्यं युगम । परजायै मर्त्यवे तवत पुनर्मार्ताण्डमाभरत ॥
Ashau putraso aditerye jatah tanvasa pari l devanupapraita saptabhih para martandam syat ll
saptabhih putraih aditirupa paraita purvyam yugam l parajayai martyeva tavata punah martandam abharat ll
Eight are the Sons of Aditi who from her body sprang to life, with seven she went to meet the Gods she cast Martanda far away.
So with her Seven Sons Aditi went forth to meet the earlier age, she brought Martanda thitherward to spring to life and die again.
Rig Veda: Rig-Veda, Book 10: HYMN LXXII. The Gods. Verse 8 & 9.