"Prema" means the affinity, the love; this love can be towards anything but complete love should be always towards Bhagavan – Paramathma; every Jeevathma has to have this attachment only towards the Paramathma but due to various karmas, the love towards the supremacy is not completely shown.
There are so many incidents where great Aathmas have shown their complete affection and affinity towards Sri Krishnar, the ultimate supremacy.
Among various incarnations, Sri Ramavathar and Sri Krishnavathar are said as the "Poorna Avathars" - the Complete Avathars.
Perumal is referred as the complete love. His Bhakthi is termed as complete love. The one who thinks of Him and shows His love to Perumal is called as "Bhakthan" every Jeevathma is a Bhakthan in any of the way; great rishis, yogis, people have felt this and they have showed their love towards Sri Krishnar.
Through love only, we can attain love. Bhakthi is the only way through which we can attain Him.
All the gopikas who lived along with Sri Krishnar worshipped Kannan and their heart keeping Himself on thier thought always - here Kannan is completely filled up on their heart. They saw Kannan on all the things; this is one of the greatest thought that every Jeevathmas should have. If we see Kannan in everything, we'll not see any difference and as a result, we see all as one and equal. This thought was very well explained by Sri Krishnar Himself through Srimath Bhagavath Gita. - "See Everything in Me. See Me in Everything"
Our belief towards Perumaan is Bhakthi and our Bhakthi towards Him, makes us to reach towards Him.
"Sri Krishnar" means "attracting everyone towards Him" [Karshathithi Krishna]. He seeds Bhakthi in our heart and he Himself makes it grow.
Before the Mahabharata war, both Pandavas and Gauravas approached Sri Krishnar to be on their side.
Pandavas had a great faith on Sri Krishna where as Duriyodhanan did not have much of faith on Him.
Sri Krishnar gave both of them a choice either they wanted a huge troop of warriors (or) only they wanted Sri Krishnar Himself along with them. Without knowing the power of Sri Kannan, Duriyodhanan wanted the troop of Sri Krishnar instead Sri Krishnar whereas the Pandavas wanted Sri Krishnar to lead them. This is Bhakthi.
Other great Bhakthi is well explained by Vidhurar, who is one way a brother to Dridharashtarar. Before the war, Sri Krishnar was invited to the palace of Gauravas. Instead of visiting their place, Sri Krishnar went towards the house of Vidhurar. Vidhurar invited Him to his small hut and placed a small wooden seat. Before the Perumaan sat on it, Vidhurar with his soft hand checked by touching the seat. Sri Krishnar questioned why he is during so? Vidhurar answered "though I know you are the God Himself, I should have not placed any nails on your seat as I live along with Duriyodhanan". As saying so, he offered Perumaan with the banana skin instead of the fruit, Bhagavan knowing the Bhakthi, accepted it, after realizing and coming to his original thought, Vidhurar felt sad for what had happened and offered Him with the fruit. This is Bhakthi.
There are so many great Aathmas who have felt themselves attracted to Perumal is through their Bhakthi.
The complete structure of Bhakthi is Sri Krishnar, to realize Bhakthi. Love towards Bhagavan is the way. Our strong belief, dedication leads us to His divine feet. Ego, anger are the bad Gunas that makes us to stop going towards Him. So, just leave these, show a great Bhakthi and we too can become as gopikas, etc.