Thoughtful

這個字基本上有兩個意思
一是沉思的；
比如說 fall into a thoughtful mood
就是陷入沉思的意思。
相似的字有meditative, wistful, contemplative... 等等

另一個更常用的意義
是體貼、為別人著想的；
也就是considerate, caring, attentive, solicitous...
反義字是thoughtless, inconsiderate, selfish。

We should always be thoughtful of people, especially those close to us.