A light-hearted traveller arrives on the bank of a river. They see two boats docked at a landing. One is laden with various cushions, refreshments, and amenities. The other is thin, streamline, and furnished with only the essentials.
The traveller can see the benefits of crossing the river in each boat. One will be comfortable and enjoyable and the other will be smooth and efficient. Wanting the benefits of travelling in both boats, the traveller decides to try to cross the river using the two of them. They untie the boats, place one foot in each, and start their journey. No sooner, however, did they encounter a few waves than they found themselves swimming in the water, having fallen through the gap that quickly opened between the boats.
The eternal soul is the traveller, the bank of the river is human birth, and the river is human life. The far shore is the fulfilment of our desires.
The boats the traveller finds are the pursuits of material happiness and spiritual happiness (the paths of preya and sreya). These are otherwise known as endeavouring to please ourselves through self-serving actions and endeavouring to please the Divine through selfless service to Krishna, Sri Guru, and all who carry love for the Divine in their hearts.
If we are resolute about attaining material happiness, we will attain that as far as it is possible (though we will discover soon enough that the limitations are quite severe). If we are resolute about attaining spiritual happiness through selfless service to the Divine, we will attain that also as far as it is possible (and discover that the potential is infinite and ever-new). If we are not resolute and try to attain both, however, we will end up with neither just like the traveller who failed to reach the shore (the fulfilment of their desires) and instead ended up falling into the river: undergoing hardship and being forced to choose which boat they would grab onto for shelter.
The moral is that sincerity is indispensable on the spiritual path.