I have a roommate who doesn't consider how his actions affect others, although it is not malicious. Whenever something is brought up he is apologetic and tries to change his ways. I'm wondering if there is a succinct word or phrase that describes this behavior?
I believe the word inconsiderate has the unconscious nature of it hardwired. You can't consciously choose to be inconsiderate, because you're considering it at that point... if you're looking for something that's a little less pejorative, then maybe "absent minded"? "Solipsistic"? ...Honestly, inconsiderate is the word you want. It doesn't have to be a put down. It's just stating the fact that he didn't consider. Never crossed his mind. Now whatever reason you give or theorize for why it wouldn't have crossed his mind might be negative, but the word itself I don't believe is, in this context.
Inconsiderate or Thoughtless
'Inconsiderate' is exactly the right word for unconscious behaviour like this.
Most people aren't actively trying to be self-centred.
People don't generally weigh up the consequences of their actions.
In my experience, an average person will stop in the middle of a crowded street to answer their phone, inconveniencing everyone who was walking behind them.
Not because they don't care about people behind them, but because they didn't think about it.
It's a kind of social clumsiness. They don't do it intentionally.
They simply don't think about or or consider it.
Thoughtless or Inconsiderate accurately describes this behaviour.
I think you are trying to distinguish someone who doesn't care about others from someone who simply struggles or forgets to think ahead in relation to others.
This is quite a tricky one. The concepts are better distinguished in some languages.
(And contrary to another answer I think that, in informal language, someone who knows full well that he/she is inconveniencing others can be called inconsiderate—e.g. someone who eats half the cupcakes but doesn't care.)
I think various near-synonyms like unthoughtful, thoughtless, unthinking, careless, self-absorbed, self-seeking would have slightly different connotations in different contexts so perhaps one will work for you. (Thanks for some of these @Decapitated Soul.)
A few phrases: "in his/her own world", "a bull in a china shop", "doesn't stop and think". Or just something descriptive like "he forgets about others", "forgets to think about others", "inconsiderate but means well", "not great at considering people".
Tactless: not careful about saying or doing something that could upset someone; someone who is tactless is not careful about the way that they speak or behave towards other people and so often upsets them.
Definition of selfish
1: concerned excessively or exclusively with oneself : seeking or concentrating on one's own advantage, pleasure, or well-being without regard for others
2: arising from concern with one's own welfare or advantage in disregard of others a selfish act
Discovering the correct 'diagnosis' is a good beginning if one cares and is willing help the person discover the underlying roots of the problem. The person may be open to exploring the issue, or uses an apology to as a learned response to 'get passed the rough patch'.
Self-seeking: Having concern for one's own welfare and interests before those of others - Lexico
Example: A lot of people feel that politicians are just self-seeking opportunists.
Unthoughtful: Showing little or no consideration for the needs of other people; thoughtless - Lexico
Example: How could she be so unthoughtful as to depart at such an awkward hour?
If you want something stronger than unconsidered, perhaps "apathy" is the word to describe it.
Here is a great example for it: https://whitneyhess.com/blog/2012/08/21/on-empathy-and-apathy-two-case-studies/
Unseemly: Not in keeping with established standards of taste or proper form; unbecoming or indecorous in appearance, speech, conduct, etc.