It's fairly easy to guess that Ichiro's tsundere, snappy attitude stems from his fear/unwillingness to appear weak to anyone, used as a way to mask his self-doubt.
However, around those he's most comfortable with, primarily those he ends up in a relationship with, he is more likely to let this mask drop (duh).
And, with this, comes one of the downfalls of being in a relationship with Ichiro. His self-esteem issues express themselves much more frequently, meaning he can occasionally be self-deprecating and negative, which no doubt will be a pain for his partner to deal with.
Don't be surprised if he attempts to break off the relationship because he feels he is inadequet. (he's not serious, just having a moment of self-doubt.)