This week's parsha features the birth of 11 out of Yaakov's 12 sons. There is an interesting pattern that emerges – all of the children are named by either Leah or Rachel with the singular exception of Levi. Rashi (29:34) notes the anomaly and quotes the midrash that Gavriel the mal'ach was sent to name him. On a simpler, level, however, it would seem that the one who named Levi was likely Yaakov. That begs the question, however, what was unique about Levi that Yaakov named him on his own?
Rada"k explains that Yaakov was inspired by a prophecy showing the great role that Levi would play in doing the avodah and teaching Torah as well as Leah's words and that is why he named him.
If we look closely at the names of the children and the reasonings given by the imahos for each, they all seemed to reflect a personal experience of the mother – HaShem saw my affliction, HaShem saw that I was unloved, etc. Levi is an exception to this rule. Leah expressed that now that she has three children, her husband will now give her special attention. It would seem that this is what inspired Yaakov to provide Levi's name.
However, in this aspect, Levi is not the sole exception. The naming of Zevulun (30:20) is also based on Leah's wish that the birth of this child would provide her special attention from Yaakov. Yet, we do not find Yaakov doing the naming in this case.
Chizkuni addresses the naming of Levi and provides the same answer. Even better, he also identifies Zevulun as another exception and explains why Yaakov did not provide the name in that case. When Leah expressed her certainty that Yaakov would be more attached to her following the birth of Levi, Yaakov concurred and so he named him based on that statement. However, when Leah expressed her thoughts following the birth of Zevulun, Yaakov was not in agreement so he did not provide the name in that case.