西瓜满了夏天我却丢了少年
[xī guā măn le xià tiān wŏ què diū le shăo nián]
Here, '西瓜满了夏天(means watermelon full summer)' indicates a perfect or ideal summer; but after summer, he seems to lose something or someone important during growing up or change into adult, such as lost his youthfulness or friends.