Formal:
* Concerned: This is the most direct opposite, meaning to be worried or anxious about something.
* Interested: This implies a positive feeling towards something, suggesting a desire to know more or be involved.
* Attentive: This focuses on actively paying attention to something.
* Engaged: This suggests a willingness to participate and be involved.
Informal:
* Worried: This is similar to "concerned," but often carries a stronger emotional weight.
* Anxious: This implies a feeling of unease or nervousness.
* Stressed: This implies a feeling of pressure or overwhelm.
* Invested: This suggests a strong personal stake in something.
Other possibilities:
* Aware: This implies a conscious understanding of something.
* Sensitive: This implies an ability to be affected by something.
* Empathetic: This implies the ability to understand and share the feelings of others.
The best opposite word will depend on the specific situation and what you are trying to convey.