The former Leeds winger made the switch on Saturday.
He told whufc.com: "I spoke to the Technical Director and to the Head Coach, and I just felt very positive about this move.
“He (Julen Lopetegui) told me that he remembered me from when I played against him, and that he was an admirer of me. He said he wanted to work with me, and that he thought I could make a contribution to the team, which was obviously nice to hear.
“He also said that he thought he could help me with my development, which was good. He is a straight talker, which is something I really like, because it means I know what he expects of me, and after speaking to him I was sure he was the right person to help take my career to the next level.”
Summerville also said: “I’ve played at London Stadium twice before, and it’s really nice. The fans always stick by their team, and there’s so much history at the Club as well.
“I obviously know about the UEFA Europa Conference League win in 2023, and I’ve seen how well West Ham have done in Europe over the last few years. The team’s plan is to push to do that again, and I’m very confident in the project here. For the Club and for my career, I’d love to play in Europe for the first time.”