It's understandable. Warren is probably the most popular and recognizable Democrat in the country apart from President Obama, Joe Biden, and Clinton herself. She's proven to be a very effective and eager anti-Trump campaigner, making headlines for her attacks on the Donald's business record, for sparring with him on Twitter, and most recently for a speech accusing the GOP nominee of exploiting the financial crisis for profit

Outside of Warren, Housing and Urban Development Secretary Julián Castro gets the most buzz, but he's laughably unqualified. He hasn't done much of note at the Department of Housing and Urban Development in the past two years, and he served as San Antonio mayor when that was a part-time job paying $3,000 a year plus $20 a council session; San Antonio uses a council manager system, where the mayor is basically a glorified city councilor.

Sen. Tim Kaine (D-VA) is in a swing state and speaks better Spanish than Castro from his time as a missionary in Honduras, but he also has a strong anti-abortion record that wouldn't be a great addition to the first woman-headed major-party ticket in American history.

Labor Secretary Tom Perez is popular with liberals, but Clinton probably wants a running mate who's won a more recent and notable election than the 2002 county council race in Montgomery County, Maryland.

Former Sen. Evan Bayh (D-IN) — perennially on Democrats' VP shortlist, including in 2008 — has ruled himself out by spending the past six years as a lobbyist. Not so attractive in a year when a democratic socialist decrying big money in politics ran a strong campaign against Clinton.

Sen. Sherrod Brown (D-OH) could help shore up Sanders supporters, but he's a key senator in a swing state whose replacement would be picked by a Republican governor. He's also not a national figure the way Paul Ryan, Joe Biden, John Edwards, or Dick Cheney was.