A good response to someone saying, "I am happy to have you in my life" would be heartfelt and appreciative. Here are some possible responses:
"Thank you so much! I feel incredibly blessed to have you in my life too."
"The feeling is mutual! I cherish our friendship/relationship and value every moment with you."
"You've brought so much joy and positivity into my life. I'm grateful to have you by my side."
"Your presence has enriched my life in countless ways. I'm truly grateful for our connection."
"I'm touched by your words. I couldn't ask for a better friend/partner. You mean the world to me."
"Knowing that I bring happiness to your life makes me feel incredibly special. Thank you."
"Likewise, I feel so lucky to share this journey with you. You make everything better."
"It warms my heart to hear you say that. You're an important part of my life too."
Remember, the key is to be sincere and genuine in your response, reflecting the emotions conveyed by the initial statement.