Being in body consciousness is called tamas guna. Being in mind consciousness is called rajas guna. And being in soul consciousness is called sattva guna.
Without any one of these qualities or gunas no one can exist in this world physically.
Because without tamas guna, we do not consider about our body needs. Without rajas guna, we have not been active to work for our survival and mainly no attachments will be formed with anyone. And without sattva guna, we cannot feel inner peace and eternal bliss.
So human life will be harmonious with the balance of all these three qualities.
We all know the reason to avoid non vegetarian food, but although being vegetables, why onion and garlic are avoided ? Why potatoes, carrots etc., vegetables are not being avoided even though they belong to the same family ?
Well, onion and garlic are the root vegetables with tamas in nature. Eating garlic and onion affects one's qualities or gunas inimically. It triggers the tamas guna to raise more, which leads to create an unbalance between all the three gunas. More intensity of tamas guna can cause limitless body desires and lower vibrational emotions like pleasure, ignorance, anger etc. And primarily controlling the mind becomes a hard task.
Especially in the case of garlic, has been tested and proved by the Pyramid Spiritual Societies Movement that, it is surrounded by a grey color aura. This society conducts many healing workshops based on the Seth knowledge and wisdom. People who participate in their healing workshops are strictly advised to stop eating non vegetarian food and garlic.
Stanford University privately studied and proved that garlic is a slow poison which pierce the blood-brain wall. Eventually the responding time of the brain gets reduces as it desynchronizes the waves of our brain.
From the social point of view eating onions and garlic may cause body odour. This creates irritation to the one who have them and also to the others. Eating onions also cause bad breath.
So many yogis, people on the spiritual path and a sect of people in Hindu culture (Brahmins) avoid eating onion and garlic.