This statement playfully attributes the speaker's lack of focus to their preoccupation with someone special. It humorously acknowledges the distraction caused by their thoughts of this person.
The phrase "I can't focus today and blame it on you" suggests a lighthearted and affectionate tone. It playfully assigns responsibility to the person they're addressing for the speaker's inability to concentrate.
The reason given, "because I can't stop thinking about you," underscores the depth of their preoccupation and infatuation. It's a sweet and candid way to express how this person has captured their thoughts and attention, making it difficult to concentrate on anything else. Overall, it's a charming way to convey their affection and the impact this person has on their daily life.