It is an adjective that has two meanings.
The first meaning is ”to be sweet”. It is used to express the pleasant taste of sugar, or to express attractive things.
The second meaning is ”to be over-optimistic”. It is used to describe a person who is naive, who doesn’t think too deeply about something, or who goes easy on themselves and others.
Ken and Leo are taking a break at a café.
- Ken: Why have you been helping Mike with his project all week?
なんで、今週はずっとマイクのプロジェクトを手伝っているの？ - Leo: He told me that he would hook me up with someone nice if I helped him.
手伝ったら、いい人を紹介してくれるらしい。 - Ken: Uh-huh. You were taken in by his sweet words, right?
なるほど。甘い言葉にそそのかされた(*1)んだ？ - Leo: Well, kind of. You seem really busy. Is everything okay?
まぁ、そんなところ。忙しそうだけど、大丈夫？ - Ken: Yeah. It just turned out I have to go on a business trip to Osaka in three days. So I have some tasks that I can’t do from there.
うん。三日後に大阪へ出張に行かなきゃいけなくなったんだ。それで、あっちじゃ出来ない事が幾つかあって。 - Leo: I see. But I think you can give those tasks to other people here.
そっか。でも、他の人に頼めばいいじゃん。 - Ken: Everyone has things to do. I can’t rely on others.
皆、やるべき事があるし、甘えられない(or頼れない)よ。 - Leo: I respect you. You’re always strict with yourself and generous with others.
さすが。ケンはいつも「自分に厳しく、他人に甘い(or優しい)。(*2)」ね。
Meg and Amy are talking about their friend’s marriage.
- Meg: Kumi is so lucky! She will get married to that rich guy next month! Her future looks bright.
クミが羨ましい！来月、あんなお金持ちと結婚するなんて！将来安泰だね。 - Amy: I think it’s too naive to think that if you are rich, you must be happy.
お金持ちが幸せだなんて、考えが甘いよ。 - Meg: I know, but her boyfriend is rich, great-looking, and so kind. Sounds like she's got the whole package.
そうだけど、彼はお金持ちで、カッコよくて、優しいでしょ。クミは、非の打ち所がない(*3)人を見つけたね。
*2 自分に厳しく、他人に甘い(or優しい): The saying is quite familiar to us. Sometimes we say 自分に甘く、他人に厳しい as sarcasm.
*3 非の打ち所がない: The saying is quite familiar to us. It is used to describe a perfect person/thing.