aprapya mam nivartante
"Those who are not faithful in this devotional service cannot attain Me, O conqueror of enemies. Therefore they return to the path of birth and death in this material world."
In the 1st sloka the Lord told that He would preach the best secret, as Arjuna was sans jealousy. In the next sloka He told that Bhakti yoga was easy to perform and would make the Lord perceptible. Whenever we tell the advantages of a method, it is necessary to tell the disadvantages of the others. Sri Krishna tells the defects of the other means opposed to Bhakti yoga. He advises Arjuna not to choose samsara means as that would involve rebirths. Upanishad says 'nanya patha ayanaya vidhyate tamevam vidwan amrutha iha bhavati'. There is no other way, than to meditate on God and do bhakti. Ayanaya = attaining Sri Vaikuntam, is possible only by Bhakti. Bhakti is like a super highway, smooth to travel and easy to reach destination. Arjuna then doubts who would reject this easy path and take up other difficult paths? Asya dharnasya = this means or way [ Bhakti yoga]. While commenting on Nammalwar's Thiruvaimozhi [1.7], in his Eedu, Swami Nanjeer says this path means the path that is of liking of the Lord Sri Krishna. Paranthapa = destroyer of enemies [Arjuna], ashraddha = without interest [ to attain the Lord]; there are people not interested in the Lord but in inferior worldly pleasures. Mam = Me [Sri Krishna], aprapya = not attaining, mrutyu = death like, samsaram =rebirths, varthmani = repetitive cycles. By choosing other paths and not attaining the Lord, the atman gets damaged [not physically, but in its gyana or knowledge] and involves in rebirths. Nivartante = get fully involved [ in the rebirth and accumulation of more and more papa/punya]. Four words of this sloka are important: purusha, mam, aprapya and nivartante. Purusha means those who are eligible to reach the Lord by practicing Bhakti yoga. Such people discard this smooth path to reach Him. Mam - such people lose the greatest of wealth, the Lord. Aprapya - the Lord is waiting to take that person in His fold; but that person is not realizing and chooses other benefits. Nivartante- this samsaram is very inferior and many scholars and Acharyas have explained the disadvantages; despite all these, they want to live in this and get all disadvantage. So, Bhakti is essential to reach Him.