Here's A short answer: As a devotee of Krishna and a practicing Bhakti-yogi, we don't eat garlic and onions because they cannot be offered to Krishna.
Here's a longer answer: You may know that onions and garlic are botanical members of the alliaceous family (alliums) – along with leeks, chives and shallots.
According to Ayurveda, India's classic medical science, foods are grouped into three categories – sattvic, rajasic and tamasic – foods in the modes of goodness, passion and ignorance. Onions and garlic, and the other alliaceous plants are classified as rajasic and tamasic, which means that they increase passion and ignorance and are not good for meditation.
Devotees of Krishna wear Tulasi (occimum sanctum wood) neck beads, often as three strands at the base of the neck. Tulasi beads worn around the neck indicate a devotee's surrender to the Lord; such a person is dear to the Lord.
Tulasi beads are said to protect one from bad dreams, accidents, attack by weapons, and inauspicious destinations at death. For these reasons and because she is dear to Krishna, devotees always wear Tulasi.
By the traditional Gaudiya Math standards, devotees will start wearing Tulasi neck beads and tilaka only after initiation when they have been given them by Srila Gurudeva.
Members of the Hare Krishna Movement follow ancient India’s Vedic literature, primarily Bhagavad-gita and Srimad-Bhagavatam. These teachings date back over 5000 years and are practiced by more than 900 million people today. The Bhagavad-gita also forms the basis of the world’s third largest religion, Hinduism.
Although the Hare Krishna Movement has only been established in the West since 1966, its roots extend thousands of years into the past, back into the Vedic tradition of India. The Vedas were originally a vocal tradition, but then were written in Sanskrit over 5000 years ago.
The compiler of the Vedic literature, Srila Vyasadeva, divided the Vedic knowledge into various departments of material and spiritual knowledge, entrusting his disciples with particular sections. In this way, the scriptures developed into the four Vedas, the Vedanta Sutras, 108 main Upanishads, the great Mahabharat which includes the Bhagavad-gita, and the 18 major Puranas, among other texts. Of the Puranas, the Bhagavat Purana or Srimad-Bhagavatam is described as the most ripened fruit of all Vedic literature. It is accepted by the Vedic tradition as the conclusion of Vedantic principles and understanding, and relates the pastimes and characteristics of the Supreme Lord.
The process of spiritual development described in the Vedic literature is one of a gradual process of God-realization and love of God. This wisdom has been carefully preserved and passed down through the centuries through the vehicle of the disciplic succession of self-realized teachers.
What is the purpose of life? Is it to suffer, to enjoy, or to attain to our original spiritual nature? Once we engage in genuine spiritual practices, which are the natural activities of the soul, our normal state of being can be reawakened in which we are fully satisfied in spiritual bliss. It is simply a matter of making the connection and linking ourselves to the Supreme again. This state of being is attained when one reaches the pure activities of the soul, which is devotional service to the Supreme Being. If we are simply happy with only eating, sleeping, mating, defending, like the animals than we will continue in the cycle of samsara, (repeated birth and death) and run the risk of falling down into the animal species. The real purpose of life is to surrender unto Guru Gauranga or God, if we don't we will have to keep coming back until we learn our lessons.
Who am I? You are a spiritual being or minute spirit soul (jiva atma), identifying with a gross material body, material mind, and material intellect. These various material layers create a 'false ego' covering over the soul. The soul who is originally always peaceful and blissful really has nothing to do with these false coverings, but due to his material contact from time immermorial, he identifys with these temporary bodies and suffers from the threefold miseries of material life. When we practice Bhakti Yoga gradually these material layers are destroyed one by one and in the process we awaken more and more of our dormant Krishna Conscious nature.
The sikha protects a vunerable area of the head (a chakra). It is also signfies that one is in the devotional school of thought. The mayavadis (impersonalist schools) have a shaven head with no sikha. From a scientific persepective, a person who keeps shikha attracts cosmic energy which imparts enlightment to the wearer. The shikha behind the head also puts little pressure on the back of the head which helps improve concentration, mind control, and memory. Women grow their hair long to protect the lower chakras which are more vunerable in women.
When the conch (Shankha) is blown, the primal sound of Om emerges. Om is the sound that was emanated by God before creation of the universes.
The conch (sankha) is one of the attributes of Lord Vishnu. It is said that once a demon name Sankhasura harassed the Demigods and stole the Vedas and hid himself in an ocean. Lord Visnu incarnated as a huge golden fish ie Matsya Avatar and killed the demon. After this the Lord is supposed to have blown the conch- shaped bone of the ear of Sankhasura. From this, arose the sound Om and from this sound emerged the Vedas. The conch is therefore known as sankha after Sankhasura. The conch of Lord Vishnu or Lord Krishna is called the Pancajanya. It represents dharma or righteousness, which is one of the four objectives (purusarthas) of human life. The sound of the conch also symbolises the victory of the good over the evil.
The Vedic scriptures establish nonviolence, called ahimsa, as the ethical foundation of vegetarianism and for a peaceful society. According to the Vedas, God is the Supreme Father of all creatures, not just humans. Therefore, slaughter of innocent animals is considered equivalent to killing one’s brother or sister.
Krishna devotees follow a wholesome lacto-vegetarian diet excluding meat, fish and eggs. Although it may be argued that vegetarians are guilty of killing vegetables, foods such as fruits, nuts, milk, and grains do not require killing. But even when a plant’s life is taken, the pain involved is dramatically less than that of a highly-sensitive animal such as a cow or lamb. When vege items are offered to the Lord they become karma free and the souls in the plant bodies make spiritual advancement.