Love is very difficult to define. It is similar to what a Supreme Court justice once said about something else "I can't define it, but I know it when I see it".
However, in Jewish sources love is defined as the desire to give to the other.
One of the reasons that love is expressed by a kiss is because a kiss symbolizes the desire of one person to give their very breath of life to the other. So when we have an overwhelming feeling toward another of a desire to give to that person - to give life, pleasure, warmth, security etc. - that is very likely love.
