There are many kinds of romantic relationships; this one is all about a protagonist who's crushing on someone. It may be a far-off person who doesn't know the character exists (a celebrity or someone at the office) or a person with whom they're already in a platonic relationship (their boss, a best friend's sibling, or a friend-of-a-friend). Sometimes the other person is oblivious to the protagonist's infatuation while, in some cases, it's obvious despite the character's best attempts at hiding it.

Each relationship is different, depending on the people involved, their history together, their individual personalities, and a host of factors. Here are some common dynamics—healthy, and not so much—that can accompany this relationship. Use the ideas that suit your story and work best for your characters to bring about and/or resolve the necessary conflict.

Admiring the other person from afar
Having a platonic relationship with the crush and trying to act like they aren't romantically interested
Using intrusive means to catch the crush's attention (by sabotaging their current relationship, crashing a private party, etc.)
Being so desperate that the protagonist will accept even negative or harmful attention should the crush offer it
Obsessing about the crush to an unhealthy degree (stalking them, neglecting other relationships, feeling life isn't worth living without them, etc.)

Someone outing the character and revealing their feelings for the crush
The crush becoming interested in someone else
The crush recognizing the character's interest and toying with them, sowing confusion
A change that throws the character and crush together (restructuring at work, the character being engaged to tutor the crush, them being assigned the same group project, etc.)
A scenario that puts the two at odds, such as them entering a contest where they must compete against one another
Another party pursuing the character, distracting them from the crush
New information (that puts the crush in a negative light or increases the attractiveness of the character for the crush)

A PARENT'S DIVORCE, A PARENT'S ABANDONMENT OR REJECTION, BEING DISOWNED OR SHUNNED, BEING REJECTED BY ONE'S PEERS, FALLING SHORT OF SOCIETY'S PHYSICAL STANDARDS, GETTING DUMPED, MISPLACED LOYALTY, SOCIAL DIFFICULTIES

HAVING TO BREAK SOMEONE'S HEART, A ROMANCE BEING STYMIED, HAVING TO BREAK A PROMISE, NOT KNOWING WHAT ONE WANTS, CONFLICTING INTERNAL NEEDS OR DESIRES, A LOVE INTEREST TAKING UP WITH SOMEONE ELSE, DOING SOMETHING STUPID WHILE IMPAIRED, A ROMANTIC COMPETITOR ENTERING THE SCENE, AN EX INTERFERING IN ONE'S LIFE, PUBLIC HUMILIATION, BEING MANIPULATED, BEING IGNORED OR BLOWN OFF, CONFIDING IN THE WRONG PERSON, BEING TAKEN FOR GRANTED, BEING REJECTED BY A POTENTIAL LOVE INTEREST, AN UNWANTED ROMANTIC ADVANCE, INDULGING WHEN ONE SHOULD NOT, UNWANTED SCRUTINY, EXPERIENCING A CRISIS OF SELF-DOUBT

When the crush is taboo (a sister's ex, someone the character's parents don't approve of, someone from a different caste, etc.)
Crushing on someone and learning that a friend is interested in the same person
Crushing on two different people
Desiring the crush but feeling unworthy of their love
Recognizing that pursuing an impossible relationship is keeping the character from dealing with past pain, but doing it anyway
The character's feelings fading when the crush begins showing interest
Longing for the crush but being too afraid to take steps to win them over 
Learning something about the crush that impacts the character's feelings (the emergence of a heinous flaw, learning that the crush has a history of abusing others, etc.)
Crushing on a friend; wanting more but not wanting to ruin the relationship

Bold and Timid
Flirtatious and Jealous
Observant and Reckless
Childish and Mature
Cruel and Innocent
Manipulative and Kind
Needy and Withdrawn
Pretentious and Uncouth

The protagonist changing their personality or values to please the crush
The character sacrificing positive relationships with others who express concern about the obsession
Being rejected by the crush and becoming depressed or self-destructive
Internal conflict leading to self-doubt, insecurity, and uncertainty
Missing out on healthy, happy relationships with other love interests because the character is so focused on the crush
The protagonist's platonic relationship with the crush falling apart because they each want different things
The protagonist missing out on a romantic relationship with the crush because they're too afraid to express their true feelings

A situation that make the character realize the crush is better as a friend or ally than a romantic partner
Being paired in a contest or competition where the character must put aside personal feelings to work alongside the crush
The crush having feelings for someone close to the character, and the character realizing they make a better pair and helping him/her win the person over
A life-threatening scenario where survival is imperative and there is no room for romantic feelings or entanglements

A better match coming along, enabling the character to embrace happiness with someone else
The character realizing that it's better to be alone than with someone who isn't a good match
An obsession revealing internal flaws that the character must work through before pursuing a healthy relationship
The protagonist stepping out of their comfort zone to make their feelings known and pursue a deeper relationship with the crush
The character choosing to cut their losses and focus their energy and resources on more fruitful endeavors

BETRAYAL, COMING OF AGE, ENSLAVEMENT, FREEDOM, FRIENDSHIP, HOPE, INNOCENCE, INSTABILITY, ISOLATION, LOVE, PERSEVERANCE, SUFFERING, VULNERABILITY