他说西瓜喜欢夏天
[tā shuō xī guā xĭ huān xià tiān]
It creates an whimsical scene using personification as if 'the watermelon said it loves summer'. This phrase seems to carry a light-hearted and playful tone which could mean the person's life view is positive or this nickname simply conveys nostalgia for good summer times associated with enjoying watermelon.