To understand the answer to all these questions we need to understand the Science of love. Following are the two main theories that govern the science of love , attraction or attachment.

Evolutionary Need

It can be considered to be the central factor that attracts the potential mate to a creature in the nature. The same is true for human beings as well. As the evolution theory goes, if a creature mates with a weaker mate, the off-springs are also weak and may not be able to survive against this harsh nature.
This might at some point of time lead to the total extinction of the species from earth. In order to avoid this, it is natural that we try to attract and also get attracted to a stronger, healthier and attractive mate so that there is no problem in the continuity of our heritage.
Though this theory might not play in our mind in our real lives but it is actually what the science of evolution tells us and it does explain to a good extent the laws of attraction and love.

The Role of Hormones

Apart from the evolution theory, this is the basic scientific reason which explains the questions we are trying to answer in this article.
Hormones are a form of chemical compounds in our body which is released at various stages of life and times as per need. The role that they play in influencing our minds and bodies to get attracted to somebody is also pivotal. A famous study defines the first stage of love to be lust, the second as Attraction and third as Attachment. These various stages are influenced by different hormones in our body which is discussed below.
Stage I - Lust
This is the first stage of love, in which we try to get a desirable being of opposite sex and satiate our carnal desires. This stage is mostly related with the sexual fantasies that a human being carries in his/her mind. The sex hormone that influence or cause this stage is testosterone and estrogen in both males and females respectively.
Stage II - Attraction
This is the most beautiful stage when one is said to have "fallen in love". The person in totally in thoughts of his/her lover and can think of nothing else that person. The hormones that play their role for such a emotional state are Dopamine and Adrenaline;

Dopamine : Dopamine is a hormone that acts as a neurotransmitter. When this hormone is triggered, the individual experience the similar pleasurable feeling like that experienced when taking drugs! This brings in a wave of pleasant feelings, additional energy and focus. The need for sleep and taking rest is felt to go down. 
Adrenaline : The process of falling for some body is characterized by a rush of adrenaline in your blood. This affects your response to stress, increased heart beat and dryness of mouth- indicators of initiation of love.
 Stage III - Attachment
This is considered to the last stage of love and very much needed for a successful relationship. Scientists believe that there are two hormones which have a role to invoke this feeling - Oxytocin and Vassopressin.

How do you feel when you fall inlove? Share your experience.